Right choice is (c) 2n
To explain: Take the NFA with states {qo,q1}, alphabet Σ={a}, initial state q0, transitions δ(q0,a)=q0, δ(q0,a)=q1 and final state q1. It generates the same language as the DFA with the same set of states and alphabet, but transitions δ(q0,a)=q1 and δ(q1,a)=q1.