Agene involved in neural development shows three times as many synonymous site substitutions than nonsynonymous ones. a likely explanation is that this gene:
a. is under positive selection.
b. is affected by heterozygote advantage.
c. has a very high mutation rate.
d. is a neutral gene.
e. is under purifying selection

e. is under purifying selection

Explanation:

Purifying selection removes deleterious alleles that can arise through

random mutations

.

Mutations  may or may not have consequences at the level of genetic expression depending on whether those mutations are synonymous or nonsynonymous. Synonymous mutations do not generate changes in the protein to be encoded while non-synonymous ones do.

Important genes for the development of the organism - such as those involved in the  neural development- are very likely to be under purifying selection, eliminating individuals with deleterious alleles since those alleles (product of non-synonymous substitutions) do not allow the individuals to survive.