An adaptation is…
a. feature of an organism that increases its fitness
b. a change in the genotypic frequencies
c. resemblance to another species
d. dependent upon close kinship

a. feature of an organism that increases its fitness

Explanation:

An adaptation can be define as the changes occurring in the organism typically in the anatomy, physiology, and at the genetic level so as to make the organism suitable for survival in the unfavorable environment. The adaptation allows the development of the features in the organisms which increases it's fitness and chances of survival.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.