The weighted average maturity (WAM) of a mortgage pass-through security is calculated by assigning weights to the remaining life of each mortgage in the pool. The weight assigned to each mortgage, denoted as $w_i$, is determined by the ratio of the remaining principal of the $i^{th}$ mortgage ($P_i$) to the total remaining principal of all mortgages in the pool. This is mathematically represented as:

$w_i = P_i / \sum_{i=1}^n P_i$

This method of weighting ensures that mortgages with larger remaining principals have a greater impact on the WAM, reflecting the fact that these mortgages will take longer to be fully repaid and thus have a greater effect on the average maturity of the security.

Choice A is incorrect. The time to maturity as a percentage of the total life of the security is not used as weights in calculating WAM. Instead, it's the remaining principal balance of each mortgage that is used.

Choice C is incorrect. The absolute value of coupon payments due does not serve as weights for calculating WAM. This would be more relevant in determining cash flows rather than the average time it will take for mortgages to be repaid.

Choice D is incorrect. While the remaining number of months to maturity for each mortgage loan does play a role in determining WAM, it's not used as weights in its calculation. Rather, these are multiplied by their respective weights (i.e., remaining principal balances) to arrive at WAM.