Choose the missing terms out of the given alternatives. H, I, K, N, ?

The correct answer is $\boxed{\text{R}}$.

The sequence is a list of letters that are two places apart in the alphabet. The first letter is $\text{H}$, which is two places before $\text{I}$. The second letter is $\text{I}$, which is two places before $\text{K}$. The third letter is $\text{K}$, which is two places before $\text{N}$. The fourth letter is $\text{N}$, which is two places before $\text{R}$.

The other options are incorrect because they are not two places before the letters in the sequence.