Comment by jjav

A couple of these are easy to answer.

A - Never. Cash with no return is just decaying with inflation.

B - Depends on interest. If your mortgage interest is lower than no-risk interest (e.g. SGOV) then no, you'd be throwing money away by paying off the mortgage. If OTOH your mortgage interest is substantially higher than no-risk interest, then yes, paying it off (or at least overpaying monthly) is a good idea. There is a gray area if your mortgage interest is slightly higher than no-risk return right now. Numerically it makes sense to pay the mortgage, but there is also a safety aspect in keeping the liquid cash on hand.

So that leaves some combination of C & D. The best ratio there is a harder question to answer.