What is a prefix for necessary?

There isn't a specific prefix that directly means "necessary" in English. However, depending on the context you want to convey, you could use prefixes like:

* in-: This prefix means "not" or "lacking" and can be used to create the opposite of "necessary," like "innecessary."

* un-: This prefix also means "not" and can be used to make the word "necessary" into "unnecessary."

* pre-: This prefix means "before" and can be used to create the idea of something being essential before something else, like "prerequisite" (something necessary before something else).

* re-: This prefix means "again" and can be used to emphasize the importance of something, like "required" (needed again).

Ultimately, the best prefix to use will depend on the specific meaning you want to convey.