Whenever an argument's conclusion feels like a bigger claim than its evidence can support, the gap is usually filled by an unstated assumption, which is exactly the pattern here.
Since the argument's conclusion only holds up if we accept this unstated assumption, and the argument never actually states or defends it, this fits the description of an argument built on a hidden assumption, rather than ambiguous definitions, circular restatement, or analogy to another case.
So the correct answer is option C.