Adominant allele is best defined as

a. the healthy allele sometimes referred to as the wild type allele
b. the stronger of two matched alleles
c. the allele that is most frequently expressed in a given population
d. an allele that stops or hides the expression of another allele

The correct answer will be option-D

Explanation:

Alleles are the different version of the genes present on the specific position called locus on a chromosome.

In a diploid organism, two versions of the same gene are found as the two copies of the chromosome are present. The trait is expressed in the organism only when these two versions come in complementary position during fusion of chromosomes.

Out of these two, the allele which shows it effect by masking or hiding the expression of the allele is known as dominant allele whereas which gets masked by the dominant allele or which is unable to show its effect is known as a recessive allele.

Thus, option-D is the correct answer.