L = {a2, a3, a5, a7, a11, a13,....}*
L will generate $\varepsilon$
L will definitely generate all even powers of a because of {a2}*
L will generate a3
Now, take any other power of a, it will definitely be generated by L. Eg a6 = a2a2a2
a9 = a7a2 or a3a3a3
So, L generates all powers of a except for a itself.
Language generated by L can be given as {a}* - {a} which is nothing but regular