The outermost electrons of the element vanadium (atomic #23)
are in which energy level of the atom? how do you know?

The outermost electrons of the element vanadium (A.N.=23) will be in forth energy level (n=4) of the atom.

We know, the capacities of orbital as K,L,M,N = 2,8,8,18 respectively, as 23 is larger than the sum of 2,8,8 it will be in next higher orbital which is forth.

Again, In short, Your Answer would be Forth Energy level

Hope this helps!

The correct answer is b