(D) 100% tall with round seeds
When TTRR × ttrr are crossed, all F₁ offspring receive one T and one R allele from each parent, giving genotype TtRr — tall (T dominant over t) and round (R dominant over r).
In a dihybrid cross between two pure-breeding parents, all F₁ offspring are heterozygous (TtRr). Since tall (T) and round (R) are dominant traits, 100% of F₁ plants show both dominant phenotypes. Examiners expect you to identify dominance and apply it directly — no Punnett square needed for this straightforward cross.