In which of the following conditions would maternal serum alpha-fetoprotein values be the highest:
Down's syndrome
Omphalocele
Gastroschisis
Spina bifida occulta
Friends, many questions are asked on serum alpha fetoprotein, therefore basic knowledge of this protein is quite vital.
Alpha Fetoprotein:

It is a glycoprotein synthesized by the fetal yolk sac in the early weeks of gestation and by the gastrointestinal tract and liver later.
It is the most abundant protein in the fetal serum.
It circulates in fetal serum and passes into fetal urine and amniotic fluid.
Concentration of AFP increases steadily in fetal serum till 13 weeks, (3 mg/ml) after which the level rapidly decreases throughout the rest of pregnancy.
AFP level in fetal serum declines following birth and by one year of age, its concentration is 1 ng/ml which persists throughout life.
AFP passes from the fetus to amniotic fluid when fetus passes urine.
It passes into the maternal serum by diffusion across the placental membranes and via placental circulation and is found in steadily increasing quantities in maternal serum after 12 weeks.

These are the usual ways of entry of serum alfa fetoprotein in maternal serum but serum alfa fetoprotein can find its way in maternal serum in other ways too.

Open fetal body wall defects uncoverd by integument permit allows additional AFP to leak into the amniotic fluid and thus maternal serum AFP are increased. This is the reason for increase in serum alpha fetoprotein in neural tube defects and ventral wall defects.
Maternal screening is done between 15-20 weeks (according to Williams 23/e, p 289, Fernando Arias 3/e, p 58).
It is measured in nanograms per ml and reported as a multiple of the median (MOM).
MSAFP of 2.5 MOM is considered as the upper limit of normal (for twin pregnancy it is 3.5 MOM).

Now after having this basic knowledge lets have a look at the question.
In the question alpha fetoprotein will be increased in the following conditions.

Gastroschisis – Ventral wall defect
Omphalocele – Ventral wall defect
Spina bifida occulta – Neural tube defect

In Down’s syndrome - AFP levels are decreased.
Spina bifida occulta:
Spina bifida occulta is usually a small, clinically asymptomatic defect, covered by skin, so there are less chances of mixing of fetal serum and maternal serum. Therefore the maternal serum alphafetoprotein level usually does not increase in spina bifida occulta.
“In the fetus with a defect such as anencephaly or spina bifida, AFP enters the amniotic fluid in increased amounts, leading to higher levels in the maternal serum as well. Levels of AFP are elevated in amniotic fluid and maternal serum only when such lesions are “open,” i.e., when the neural tissue is exposed or covered by only a thin membrane. When NTDs are skin-covered, AFP does not escape from the fetal circulation, and such defects are generally not detected by maternal serum AFP (MSAFP) screening”.
USG in Obs. and Gyane by Callen 4/e, p 25
Omphalocele: 
Fernando Arias 3/e, p 84
It is a midline defect of the anterior abdominal wall characterized by herniation of the abdominal viscera into the base of the umbilical cord.
The protruding organs are typically covered by a thin aminoperitoneal membrane. Omphalocele has a strong association with high levels of maternal serum alpha-fetoprotein because the ventral wall defect allows mixing of fetal and maternal circulation.
Gastroschisis:
Fernando Arias 2/e, p 83
Gastroschisis is a paraumbilical defect of the anterior abdominal wall, through which abdominal viscera herniates. The defect is usually located on the right side of the cord insertion and compromises the full thickness of the abdominal wall. There is no sac or membrane covering the herniated organs. This defect is associated with high alpha feto protein titre.
Both gastroschisis and omphalocele are ventral wall defects containing abdominal organs and both are associated with high alphafetoprotein level in maternal serum.
But it is likely that alphafetoprotein level will be higher in patients with gastroschisis as there is no sac or membrane which covers the herniated organs in this defect. So there is more possibility of fetoprotein leak into the maternal serum or amniotic fluid in Gastroschisis. The answer is further supported by the following graph from
USG in Obs. and Gynae by Callen 4/e, p 28