Which of the following terms means to criticize or speak about negatively and at length?

The term that best means to criticize or speak about negatively and at length is "belabor." It refers specifically to dwelling on or overemphasizing a point or argument, often to the extent that it becomes tedious or excessive. In discussions or debates, when someone belabors a point, they are not just expressing criticism but are doing so in a prolonged manner that can detract from the overall conversation. This practice can lead to frustration among listeners, as the focus is drawn away from other important aspects of the discussion.

While other terms also imply some form of criticism, they do not carry the same connotation of extended or repetitive discussion. For example, reproach typically conveys a sense of blame or disappointment without necessarily indicating a lengthy criticism. Denounce refers to a public declaration of criticism or disapproval, but it usually doesn't imply the detail or length associated with belaboring a point. Censure involves formal disapproval or condemnation, which can occur in concise terms rather than an extensive critique.

In essence, "belabor" encapsulates the idea of lengthy criticism more fully than the other terms, making it the most suitable choice.