Prefixes are important in Russian, since many verbs contain at least one. Ignoring variants (such as изо and ис for из), I count 24. (Wikipedia counts 23.) Remembering which verb begins with which prefix is challenging, since in some cases, the prefixes overlap in meaning. We could profit from a category under “Grammar Challenges” where the cloze words are all verbs that contain prefixes.
Wikipedia’s list of 40 verbal prefixes, including variants:
https://en.wiktionary.org/wiki/Category:Russian_verbal_prefixes
That page contains links to each of the prefixes.
My list of the 24 verbal prefixes when variants are excluded:
без-
в-
воз-
вы-
до-
за-
из-
на-
над-
недо-
низ-
о-
от-
пере-
по-
под-
пона-
пре-
пред-
при-
про-
раз-
с-
у-
Some of these prefixes can also be used with nouns or adjectives, but I think it would be better not to include them in the category.