Mendel crossed a pure wrinkled seeded plant with a round seeded one. all the offspring were round seeded. which trait is dominant?

According to this problem, this is an example of complete dominance, so the dominant allele would be the allele that codes for physical trait for roundness of the seed.

So the cross would be like this:
W = allele for round seed
w = allele for wrinkled seed

P1 gen = ww • WW
F1 gen = Ww
Since all of the offspring are heterozygous the result is that the dominant allele suppressed the recessive allele for wrinkled seed.