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:
So, filling of 3d orbital is represented by transition metal of 4.