Given regular expression can be written as:
$L=(a+b)(a+b)=(a+b)^2$,which accept all the strings of $a,b$ whose length is exactly $2.$
so $L=(a+b)^2=(\ aa,\ ab, \ ba,\ bb)$
Option A) wrong because it generates one length string like $a,b$ and $ba,bb$ is missing.
Option B) same explanation as A, and $aa,ab$ is missing.
Option C) wrong, produces one-length strings.
Option D) correct.
So Option $D$ is correct.
Note: in regular expression $+$ or $|$ sign represent concatenation. Concatenation provides a choice between strings.