Pure tall plants are crossed with pure dwarf plants. In the $F_1$ generation, all plants were tall. These tall plants of $F_1$ generation were selfed and the ratio of tall to dwarf plants obtained in $F_2$ was $3:1$. This is called as

The correct option is A Dominance.
As per Mendel’s law of dominance, “In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

In the given example, pure tall plants have gene TT ( which express height ) and the dwarf plant has gene tt. When they have crossed the gametes with t and T are formed. After fertilisation, the zygotes formed have Tt genes, which express as tall plants. So, if there exist two contrasting traits, one of the traits will always suppress other, thereby expressing itself. Such a trait is known as the dominating trait. The suppressed trait is known as the recessive trait.

When the Tt plant is self-fertilised, the gene expression is TT, Tt, tT and tt. The only tt is dwarf plant and all other plants are tall giving ratio of 3:1.

Thus, the correct answer is option A.