Answer:
D. Only Gg
Step-by-step explanation:
We have a plant with genotype GG and another with genotype gg. We need to make a Punnett Square (see attachment). Basically, we're putting the two alleles of Parent 1 (G and G) at the columns and the two alleles of Parent 2 (g and g) at the rows. Then, we "cross" them.
Notice from the Punnett Square that the only possible genotype of the offspring is Gg. The answer is D.
Hope this helps!