The use of a colon in this sentence is correct because a) colons can be used in place of a period. b) colons and semi-colons are interchangeable. c) colons separate two independent clauses into sections in the sentence. d) colons should be used after independent clauses that come before lists.

The use of a colon is correct because D) Colons should be used after independent clauses that come before lists.

An example of this is: My favorite colors are: blue, green, and purple.

I hope this helps you! :)

Here are the answers to the given sentences above:
-No one answered the ad but me. It is used as PREPOSITION.
-There is but one choice to make in this case.  It is used as an ADVERB.
-Sadie said she would go but she had to bring her sister. It is used as a CONJUNCTION.
Hope this helps.