The correct answer is 50%, or 1/2. We know that round seeds are dominant, since the round pea plant has a genotype of Rr and the wrinkled pea plant has a genotype of rr. If it helps, draw a Punnett square with the two genotypes.
R r
r
r
The square works exactly like a coordinate grid. Combine the letters above and beside whichever of the four spaces you chose to find the genotype for that space.
R r
r Rr rr
r Rr rr
Therefore, 2 out of 4 of the pea plants have round seeds, since a genotype of Rr means that a plant will have round seeds, and 2 out of 4 will have wrinkled seeds, since a genotype of rr means that a plant will have wrinkled seeds. 2/4 simplifies to 1/2, or 50%. Therefore, there is a 50% chance that the offspring will have wrinkled seeds.
Hope this helps!