During his experiments, Mendel crossed pure strains of pea plants to obtain the first offspring. This means that the traits were defined by a homozygous genome, e.g., the yellow color is given by AA and green by aa.
Therefore, the first cross and the resultant offspring would be as follows
Mendel saw that the green trait disappeared during the first cross, but after crossing two individuals of the offspring (second cross) he saw that the trait appeared again.
Therefore, the answer would be B