The Long-Run Cumulative Dynamic Multiplier A $X _ { t - r }$

Short Answer

The long-run cumulative dynamic multiplier a. cannot be calculated since in the long-run, we are all dead.
b. is the sum of all individual dynamic multipliers.
c. is the coefficient on $X _ { t - r }$ in the standard formulation of the distributed lag model.
d. is the difference between the coefficient on $X _ { t - 1 }$ and $X _ { t - r }$ .