Answer:
The sentence that is capitalized correctly is the following one: Only one solution is possible: We must cut our expenses.
Explanation:
A colon instead of a semicolon may be used between independent clauses when the second sentence explains, illustrates, or expands on the first one. As regards the capitalization of the first word after the colon, punctuation rules state that it depends on the style. If you are following the APA style, you should capitalize it if it begins a complete sentence, just as the example above shows.
