My answer always included the qualification that "genii" is limited to a particular meaning of "genius". If you use the word to mean "an intelligent person" then the plural can only be "geniuses".
grammar - What is the plural of the word "genius"? - English Language ...
Genius is the correct spelling that comes directly from Latin: word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus.
Why is "genius" often misspelt as "geniOus"? What are its etymons ...
On the surface, one might think that ingenious is somehow based on the word genius. Interestingly, this is not true. The word ingenious does not actually have the in- prefix for negation. Instead, it comes originally from the Latin ingeniōsus, which means "intellectual, talented, ingenious". At times in history it was also spelled "engenious". Indeed, ingeniōsus appears to be the same root ...
The simple answer is, yes. This usage of 'genius' as an adjective is relatively new and would still be considered by many (including me) as slang. In that vein, the following would also be acceptable: my genius girlfriend Your plan is genius! However, it would be more standard (and advised in formal settings) to use the word 'ingenious' in all of these examples, including your own, as it is an ...
It is perfectly correct and grammatical. "genius" as a concept (of brilliance, inventiveness, etc.) predates genius as a person (one gifted with genius). And one of the early meanings of "to answer" is to rise up to the challenge.