Answer: $\mathbf{(D)}$
As the above Regular expression is the $\mathbf{UNION}$ Of two Regular Expressions in which the first one starts with a and ends with b followed by any number of a and b in between.
In the second part, it must start with b and ends with a, followed by any number of a and b in between.
On checking the options we can easily eliminate a, b and c as they both start and end with the same symbol.
$\therefore$ The answer is $\mathbf D$