Which crossing type involves one pair of contrasting traits?

A monohybrid cross is the correct choice because it specifically involves the study of one pair of contrasting traits. This type of genetic cross focuses on how the alleles of a single gene interact when inherited from two parents. In a typical monohybrid cross, one parent contributes a dominant allele while the other contributes a recessive allele, allowing for observation of the dominant trait in the offspring.

In contrast, a dihybrid cross involves two pairs of contrasting traits, which means it examines two different genes and their interactions simultaneously. The test cross is designed to determine the genotype of an organism by crossing it with a homozygous recessive partner and does not focus on contrasting traits of a single gene. A back cross, or backcross, typically involves breeding a hybrid organism back to one of its parents, again involving more than one pair of traits if considering the dominant and recessive characteristics.

Consequently, the focus on one pair of traits and the basic principles of inheritance depicted in a monohybrid cross make it distinct and the correct answer to the question.