Jean and sean are dizygotic (fraternal twins whose mother drank alcohol throughout her pregnancy. jean shows symptoms of fetal alcohol syndrome, but sean does not. the likely explanation for this difference between the twins is that:

Fetal alcohol syndrome classifies as a toxic effect on the embryo caused by a substance during pregnancy.

The substances that cause birth abnormalities or deffects are called teratogens.

The teratogens may have different consequences on the individuals (in these case the twins) depending on their genetic makeup.

So, the answer is that the likely explanation for this difference between the twins is that a teratogen may or may not be harmful depending of the genetic makeup.

I believe that the answer is D.