Answer:
A. All short plants are homozygous for that trait
Step-by-step explanation:
Although there is no visible punnet square but the question is very straightforward. The gene involved here is that coding for height in pea plants. The tall allele (T) is dominant over the short allele (t) i.e. the allele for tallness will always mask the expression of the short allele in the gene.
Due to this principle of dominance, the trait for tallness can either be homozygous, TT (same allele for tallness) or heterozygous, TT (different alleles resulting from the combination of tallness allele and shortness allele). Note that, this heterozygous state is possible because of the dominance the tallness allele (T) has over the shortness allele (t).
However, the recessive trait which is shortness can only be expressed in a homozygous state i.e. when the allele for shortness are the same in the gene i.e. tt