Answer:
3d
Explanation:
Transition metal are elements having partially filled d sub shell. In these metals last electron is filled in d sub shell.
General electronic configuration of transition metal: 
Where, n = outermost shell.
Period 4 means, last shell no. is 4.
For transition metals of period 4,
General electronic configuration: 
for example, Fe belong to periods 4.
Electronic configuration of Fe: ![[Ar]3d^64s^2](https://tex.z-dn.net/?f=%5BAr%5D3d%5E64s%5E2)
So, filling of 3d orbital is represented by transition metal of 4.