Answer : The lowest numbered energy level where a 'd' sublevel is found is 3.
Explanation : According to the theory of order of electron filling;
The 4s orbitals is observed to have a lower energy than the 3d, and so the 4s orbitals are filled first.
It is known that 4s electrons are lost first during ionisation activity. Therefore, the electrons lost first will come from the highest energy level, furthest from the influence of the nucleus.
So, the lowest energy level where 'd' sub shell will be found is 3d.