Which one of the following is the formula of weight average molecular mass$(\bar{M_w})$:-

Question

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