If the yellow allele is dominant (Y) and the green allele is recessive (y), what are the possible genotypes of a yellow offspring

The possible genotypes are YY or Yy.

Since the yellow allele is dominant over the green allele, whenever the yellow allele is present in the genotype, the offspring will show yellow phenotype. Doesn't matter whether the green allele is present or not. Only if both allele are green will make the offspring green.

Therefore, both YY and Yy genotypes will produce a yellow offspring.