Prefixes are parts of words that, when added to the beginning of a word, change its meaning in some way. They cannot always be used for every word. Instead, only a certain number of prefixes are applicable to certain vocabulary.
You also need to be careful to not mistake prefixes with parts of the root word. If you are asking about the prefixes to the word “reasonable”, there are none. Even though “re-” is a common prefix that means again, it is actually part of the root word here.
If you are asking about prefixes that could be added to “reasonable” to make it into a new word, the only one I can think of is “un”. Un- is a suffix that is negative, and using it reverses the meaning of the word.
That would make the word unreasonable, meaning something that cannot be appropriate for a situation. There are no other prefixes that can be added to reasonable and still make sense.
Note that reasonable has a suffix, or parts of the word that is added to the end! The -able at the end of the root word change the root word, reason, into an adjective, reasonable.
