What prefix would you use for the word probable?

There isn't a common prefix used specifically for "probable". However, depending on the meaning you want to convey, you could use prefixes like:

* im-: to express the opposite of probable, meaning "improbable".

* un-: similar to "im-", also meaning "improbable".

* highly-: to emphasize a high degree of probability.

* most-: similar to "highly-", expressing a very high level of probability.

* in-: sometimes used to indicate the opposite of a word, so could be used for "improbable".

The specific prefix you choose will depend on the context and the nuance you want to express.