In this question, we are looking at the ratio of pure tall plants to short plants in the F2 generation of a pea plant cross.

The cross is between a pure tall plant (TT) and a short plant (tt). The tall plant is homozygous dominant for the tall trait, meaning its genotype is TT and it can only produce tall offspring. The short plant is homozygous recessive for the short trait, meaning its genotype is tt and it can only produce short offspring.

When these two plants are crossed, all the F1 generation plants will be heterozygous for the tall trait (Tt). These heterozygous plants are tall because the tall trait (T) is dominant over the short trait (t).

In the F2 generation, the plants from the F1 generation will undergo random segregation of their alleles. This means that on average, out of every four F2 plants, three will have the tall trait (T_) and one will have the short trait (tt).

Therefore, the ratio of pure tall plants to short plants in the F2 generation is 3:1, which corresponds to option 3.