Another quick approach of solving this question for keen observers :-
Observe that $aa$ or $bb$ is minimal string that is possible in first Regular Expression $(a + b)^* (aa + bb) (a + b)^*.$
(A) We can have $ba$ or $ab$ as minimal strings which is not possible in $(a + b)^* (aa + bb) (a + b)^*$
(B) We can have empty string, which is not possible in $(a + b)^* (aa + bb) (a + b)^*.$
(D) Minimum string length is $3,$ $aa$ or $bb$ is not possible in this RE.
This rules out options A, B and D. So, option C must be the answer.