The filling of which orbital is represented by the transition metals in period 4? 3d 3s 2s 2p

Transition metals are the metal located in the center block of the periodic table. Also, this block represents the D sublevel.

So, the answer is 3d.

Also, keep in mind that the D block starts "one period late". this means that if you are in period 4, the 3d orbital starts. if you are in period 5, that where 4d starts.