Which of the following is the formula of number average molecular mass $\bar{M_N}$?

Question

$\begin{array}{1 1}(a)\;\bar{M_n}=\large\frac{\sum N_iM_i}{\sum N_i}&(b)\; \bar{M_n}=\large\frac{\sum N_iM_i}{\sum M_i}\\(c)\;\bar{M_n}=\sum N_iM_i&(d)\;\bar{M_n}=\large\frac{\sum N_iM_i^2}{\sum M_i}\end{array}$