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

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.