Final answer:
The probability of the offspring having genotype Aa is 3/4.
Step-by-step explanation:
The probability of the offspring having genotype Aa can be determined using the Punnett square method. If the parents are both heterozygous (Aa), there is a 25% chance that the offspring will inherit the genotype Aa, which is the same as Aa + aA. Therefore, the correct answer is option A. 3/4 Aa.