If you are trying to remember a phone number and you repeat it over and over again until you can write it down, you are using which technique? A. Processing B. Chunking C. Rehearsal D. Memory

Rehearsal technique

You are using rehearsal, D is an odd option to have because it encompasses all three other options in some way.
Regardless the technique you are using is rehearsal.

According to Miller (1956) you can only store about 7 bits of information in your short term memory at a time, rehearsal keeps these 7 or so items continually in your short term memory and stops them being replaced by new info.

Memory.

If you already had it written, I would say learning be rote, or by repetition. However, it seems you merely want the chance to get it written down so you needn't worry about forgetting it, or remembering it.

I'd call that the anxiety or can't trust my brain technique. Or the absent minded professor technique.

You would be using repetitive memory .