Explain why the D- block in the periodic table has only ten groups

Explanation:

The elements in groups 1B through 8B (also known as 3 through 12) are called the transition metals. ... They are called d block elements because the electrons being added in this block of elements are being added to the d orbitals.

The d-block metals are 10 columns wide due to the nature of the d-electron orbital subshell, which can hold up to 10 electrons in total, 2 per orbital.