Which of these transition words would be appropriate for indicating time?

A. again B. furthermore C. during D. besides

For indicating time, the best answer is C - During, because this word tells a reader when an action is taking place.

Again implies that something has been done before, and is being done again, but does not imply when it is being re-done.

Furthermore is a word with a similar meaning to "Also", so it is not connected to the idea of time.

Besides generally means "other than" which, again, has no connection to time.