In which one of the following sentences is but used as a preposition? a. the lake is pretty, but it's too cold for swimming. b. we discovered that candy will do anything but work. c. i wanted to leave, but i was embarrassed to do so. d. i would like to return to italy, but i have no money.

The correct answer is B.

In this sentence, "but" is used as a preposition before a noun, and the full prepositional phrase is "but work."

In the other three sentences, "but" is used as a coordinating conjunction in compound sentence. In these sentences, "but" joins two independent clauses to form a compound sentence.

It is only in sentence B that "but" functions as a preposition.