I mean… what prompted verb?
(The answer it wants is しかない, but it’s not really made clear in the grammar point writeup that it can be used without a verb, as the structure is given as: “Number + しか + Verb［ない］”)
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“nai” is a verb by itself 
(the negative form of aru) but I agree that this hint is confusing…
In this case though, the prompted verb is usually in parentheses after the sentence, so it’s pretty confusing if it’s only hinted in the translation of the sentence