Answer:
Aa
Explanation:
In Punnett Square, you are bringing the letters on the top to the numbered squares, if that makes sense. It is to predict the genotype of offspring.
So square 1, on the left is A, and on the top is A, so it would be AA.
For square 2, on the left is a, and on the top is A, so it would be Aa.
For square 3, on the left is A, and on the top is A, so it is AA.
Square 4, your question, has a on the left, and A on the top, so it is Aa.
Hope it makes sense. You should probably watch a Punnett Square tutorial online or something for a full and more general explanation.