Genes which code for a pair of contrasting traits are known as:

The correct answer is Option (2) → Alleles

Genes that code for a pair of contrasting traits are called alleles. They are slightly different forms of the same gene. In pea, for the character of height, T (tall) and t (dwarf) are alleles of each other. Alleles may be similar, as in the homozygotes TT and tt, or dissimilar, as in the heterozygote Tt. The word gives no idea of which form is dominant; it only says that the two forms belong to the same gene.

NCERT Extract:
Inheritance of One Gene – “… He called these things as ‘factors’. Now we call them as genes. Genes, therefore, are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.”