The answer is "reason"(D).
Step-by-step explanation:
This soliloquy is placed near the closing of the scene (Act IV, Scene 4). In this scene, Hamlet is out of his kingdom. There, he notices Fortinbras’ enormous army arriving to battle for a tiny and worthless piece of territory. The soldiers’ ability to fight for honor is highlighted here, which impresses Hamlet. This reasons Hamlet, a great thinker, and researcher, to think over his own state and directs him to the right path he needed to follow at that point.
This self-processing brings major changes for Hamlet. His ability to act and follow his goal enhances. Various thoughts are staged throughout this soliloquy such as philosophical reflection, his state of heart, and his nature of prominence.
Anyhow, in order to completely comprehend the phrase in the query, let us rupture the group of words. This phrase, in particular, is very meaningful and interesting in the dialogue. Here, Hamlet explains why God provided humans ability to think and reason, not to waste but to utilize it in our decisions regarding the past and future. The word "fust" is literally defined as "to decay." Hamlet applauds humans' ability to reason and understand. He pays special emphasis referring to it as a "god-like" capacity in ordinary human beings and warns not to ignore it else it will rot away and die.