Which term describes a type of trait that is usually expressed only when an organism has 2 identical alleles for the trait?
a. heterozygous < < < <
b. dominant
c. homozygous < < < <
d. recessive
im stuck between these 2.

The right type of trait is D. recessive.

The terms recessive and dominant are traits (or characters) that relate to alleles.

Homozygosity and heterozygosity characterize individuals. But in the question we are looking for a type of trait (so the answer should not be either of these two).

An allele is dominant if present on only one of the two homologous chromosomes (one of paternal origin, the other of maternal origin), it is capable of expressing a character. Thus it expresses its character that it is present on the two chromosomes of the pair or on only one. An allele is recessive if its expression requires its presence on both homologous chromosomes (in the homozygous state). A dominant allele masks the presence of a recessive allele (in the heterozygous or homozygous state).

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: