Reductions in have been linked to small declines in memory functioning and to the severe memory loss associated with Alzheimer disease.

Acetylcholine

(I sure do hope this is correct. I haven't slept yet.)

the right answer is a

there are no options provided but you can choose among the following gender-neutral pronouns: zir, hir, eir, vis, tem, eir.

in the normal course of maintaining a pronoun-antecedent agreement, we could have used the words his or her but since we are also required to use gender-neutral language, we need to replace it with its equivalent gender-neutral pronoun.