which one of the sentences uses but as a preposition?

Question

A I wanted to leave,but I was embaressed to do so.
B The lake is pretty, but it's too cold for swimming.

Neither of those sentences uses "but" as a preposition. In both it is used as a conjunction.

"But" is very seldom used as a preposition. When it is used as a preposition, "but" means the same as except. Example: No one but Andrew ate the worms.

I NEED THE NINE PROOFREADING MARKS

I don't know which nine you need, but here is a good comprehensive webpage:

http://www.journalismcareers.com/articles/proofreadingsymbols.shtml

=)