When mendel crossed true-breeding dominant yellow-seed plants with true-breeding recessive green-seed plants, the offspring were phenotypically

Yellow seed

Explanation:

When Mendel performed monohybrid crosses, he always found the F1 progeny expressing the dominant trait of the gene. In the given experiment, he crossed the dominant yellow-seed plants with true-breeding recessive green-seed plants. Since the phenotype "yellow seed" is dominant over the "green seed", all the progeny from the cross would have "yellow seed" phenotype.  

Let's assume that the allele "Y" gives a "yellow seed" phenotype while the allele "y" gives a "green seed" phenotype. The genotype of the pure breeding yellow seed plant would be YY and that of the pure breeding green seed plant would be "yy". A cross between YY and yy would give all the progeny with genotype "Yy" and would express the dominant trait (yellow seed).  

Cross: YY (yellow seed) x yy (green seed) = Yy (yellow seed)