Question

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

A. None but the brave shall enter in.
B. Children should be seen but not heard.
C. We would like to visit the Grand Canyon, but our vacation time is up.
D. John liked baseball but not football.

Answer 1

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

Answer 2

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.