A. Two or more alternative forms of a gene → Allele (III)
A gene can exist in different forms that control the same character. These alternative forms are called alleles (e.g., T and t for height).

B. Cross of F₁ progeny with homozygous recessive parent → Test cross (IV)
A test cross is specifically defined as crossing an individual (usually F₁) with a homozygous recessive parent to determine its genotype.

C. Cross of F₁ progeny with any of the parents → Back cross (I)
A back cross involves crossing the F₁ generation with either of the parents (dominant or recessive). Test cross is a type of back cross.

D. AB blood group → Co-dominance (II)
In AB blood group, both alleles (Iᴬ and Iᴮ) express equally without blending, showing co-dominance.