Which response best confirms the uncertainty about the hacker's identity?

The response that best confirms the uncertainty about the hacker's identity is the phrase "Not exactly." This phrase suggests that while there might be some partial information or assumptions about the hacker's identity, it is not definitive or clear. The use of "exactly" indicates that the speaker has some awareness of the possibility but is hesitating to assert a firm conclusion. In contexts where ambiguity and uncertainty are present, this kind of response signals a level of confusion or lack of clarity regarding the specifics.

The other options do not carry the same weight of uncertainty. "Not really" can imply a rejection of a thought or idea, but it may not reflect the same degree of ambiguity as "Not exactly." "Not anyone" and "Not someone" both suggest a definite negation of the identity of the hacker, implying that there is an absence of any particular individual, thereby lacking the nuanced uncertainty that "Not exactly" encapsulates.