ANSWER:
The formation of a positive ion is always an endothermic reaction. The reason is that positive ion is formed by the removal of one or more electrons from the neutral atom. Since, electrons are strongly held by the nucleus, so a sufficient amount of energy is required to break their attraction with the nucleus.
M ⟶ M+ + e– ∆H = Positive
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 10)