In which sentence is the word but used as a preposition?

ANone but the brave shall enter in.
BChildren should be seen but not heard.
CWe would like to visit the Grand Canyon, but our vacation time is up.
DJohn liked baseball but not football.

Verified Answer :

A. None but the brave shall enter in.

If You can substitute "apart from" in place of "but" and have the sentence still make sense, it is most likely a preposition. None apart from the brace shall enter in, is the only one that makes sense.

None but the brave shall enter.
But here means except which does the function of a preposition