Which of the following does not have a proper subset?

{x : x ∈ Q} {x : x ∈ N, 3 < x < 4} {x : x ∈ Q, 3 < x < 4} None of these

{x : x ∈ N, 3 < x < 4}

The set {x : x ∈ N, 3 < x < 4} is empty because there is no natural number between 3 and 4. ∴ This set cannot have a proper subset. Hence (2) is the correct answer.