Step 1: “Inured” means being accustomed to something, especially something unpleasant, to the point where it no longer has a negative effect.
Step 2: “Hardened” fits with the meaning of becoming less sensitive or more resilient to something over time, which is similar to being inured to an experience.
Step 3: “Softened” means to become gentler or more tender, which is the opposite of being “inured.” “Adulterated” refers to something being made impure, and “flattened” refers to making something level or even, neither of which match the meaning of “inured.”
Thus, the correct answer is: “hardened.”