Merely is a tricky word.

Watch out if you see MERELY + a blank.

MERELY means 'only' or 'just' or 'simply'. Its function is to minimize the next word.

If a sentence says that something is 'merely __________', then the word in the blank should mean something insignificant or limited.

So 'merely' an acquaintance would work. 'merely my best friend' would NOT work.