The correct answer is "enough". Since apples are countable, we would use "enough" to indicate that the quantity of apples is sufficient for the cake.
Thus, the sentence would be: "We have enough apples, that should be enough for the cake."
However, the available options are:
Out of these options the most fitting one is a lot.
So, the full sentence would be: We have a lot apples, that should be enough for the cake.