Answer
The ns orbital electrons are lost first.
Work Step by Step
When a transition metal forms an ion, it loses electrons first from the ns orbital before losing electrons from the \[\left( n-1 \right)\text{d}\] orbital.
For example, Fe has an electron configuration of\[[Ar]3{{d}^{6}}4{{s}^{2}}\] and during the formation of Fe2+ it loses both the 4s electrons.
