If it is possible to make only one meaningful English word with…

If it is possible to make only one meaningful English word with the second, the fourth, the fifth and seventh letters of the word COURTESY, using each letter only once in the word, which of the following will be the third letter of that word? If no such word can be formed, give 'N' as the answer and if more than one such word can be formed, give 'M' as the answer.

Its source gives only the correct option, without the working.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #59867