In a pair of contrasting traits, expressing traits and not expressing traits in the progeny are called as _____ and _____ respectively.

The correct option is D

dominant ; recessive

A heterozygous organism is the one having dissimilar alleles for a particular gene. It has a dominant allele and a recessive allele. So, by definition, the expressing allele is the dominant one and the non-expressing one is the recessive allele.