Number of states in minimal $\text{DFA}$ - $k$ must be $\leq 2^{n}$ as each state corresponds to a subset of states of the corresponding $\text{NFA}$.
$\text{D}$ is answer.
Option $\text{B}$ is not always $\text{TRUE}$ as the $\text{NFA}$ $N$ can have epsilon moves and hence can have more number of states than the minimal $\text{DFA}$.