chemistry multiple choice

Which statement about elements in the d block of Period 4 of the periodic table is correct?
A Cr atoms have the electron configuration: 1s22s22p63s23p63d54s1.
B Cu+ ions contain an incomplete 3d sub-shell.
C Fe2+ ions contain 3 unpaired electrons.
D Sc forms ions with different oxidation states.
can anyone help plsss- answer is A
I don't get how to work these out

You have to learn that chromium is an exception to the general rules with electron configurations (i.e it has 1 4s electron as opposed to 2, so that the 3d subshell is half-filled).

ok thank you- what's the general rule for 4s sub shell, except for cr is this always filled before 3d?

The general rule is that 4s fills fully before 3d does. The exceptions are Cu and Cr.