Question

Which energy sublevel is being filled by the elements y to cd? 4d 5p 5s 5f 5d?

Answer 1

Elements from Y  to Cd are in the 5th period, and the are transitional metals,
so, sub-level should be d , and this d sub-level, is going to be on the level that is 1 less than number of the period (5-1=4).
Answer is 4d.