Write down the number of 3d electrons in Co^2+ ion.

The correct answer is Option (1) → 7

To count the 3d electrons of a transition metal ion, write the configuration of the neutral atom from its atomic number, then remove the electrons lost in forming the ion. The 4s electrons are removed first, and only then the 3d electrons.

Co (Z = 27):

    \( \displaystyle \text{Co}: \mathrm{[Ar]\,3d^{7}\,4s^{2}} \)

Forming \( \ce{Co^2+} \) removes the two 4s electrons:

    \( \displaystyle \ce{Co^2+}: \mathrm{[Ar]\,3d^{7}} \)

So \( \ce{Co^2+} \) has 7 electrons in its 3d subshell. (Removing one more electron, now from 3d, gives \( \ce{Co^3+} \): \( \mathrm{3d^{6}} \).)