Is there a sentence fragment?Which U.S. cities are doing the most to reduce greenhouse gas emissions? Tonight's news report has the details.

Yes, there is a sentence fragment in your text. A sentence fragment is a group of words that does not express a complete thought and is often missing a subject or verb.

In your text, "Which U.S. cities are doing the most to reduce greenhouse gas emissions?" is a complete question and serves as a main clause. However, the phrase "Tonight's news report has the details." is a complete sentence that informs us about subsequent information.

The fragment in your query is the first part, which could stand alone with a question mark but is presented in a confusing manner because it is followed directly by the statement. To connect these ideas more clearly, you could revise it to: "Which U.S. cities are doing the most to reduce greenhouse gas emissions? The details will be provided in tonight's news report." This way, both parts are clearly articulated and connected.