The coefficient of $x^m$ in the expansion 1 + $(1+x)$ + $(1+x)^2$ + ..... + $(1+x)^n$ is

Question

$\begin{array}{1 1} ^{n+1} C_{m+1} \\ ^n C_m+1 \\ ^{n+1} C_m \\^nC_m \end{array} $