Answer:
The answer is D. a weapon.
Step-by-step explanation:
A blunderbuss is a type of weapon, more specifically a short-barreled gun used at short range. The guard, in the passage we are studying here, replaced his gun in his arm-chest. An arm-chest is a type of box used in the military service to transport small arms. Thus, the best choice for the meaning of blunderbuss would be letter d. a weapon.