Which of the Following Reactions Is the Most Likely to Be

Which of the following reactions is the most likely to be endothermic?

A) Mg(s) $\rightarrow$ Mg(g)
B) Na⁺(g) + Cl^-(g) $\rightarrow$ NaCl(s)
C) Na⁺(g) + e^- $\rightarrow$ Na(g)
D) H⁺(aq) + OH^-(aq) $\rightarrow$ H₂O(l)
E) 2 H₂(g) + O₂(g) $\rightarrow$ 2 H₂O(l)