Question

In which sentence is the word but used as a preposition?
A. Children should be seen but not heard.
B. None but the brave shall enter in.
C. We would like to visit the Grand Canyon, but our vacation time is up.
D. John liked baseball but not football.

Answer 1

In A, C and D it serves its usual use as a conjunction. It is a preposition in B, where it is something like "Except for".

Answer 2

B