Correct option is D
No improvement is needed as "to" is correctly used.
Explanation: The preposition "to" is appropriately used to indicate the direction or relation of the criticism being aimed at Mridula. The structure "impervious to" is a standard expression meaning unaffected by.
Grammatical rule used: "Impervious to" is used to express resistance or insensitivity to something.
Example: She remains impervious to their criticisms.