The correct option is D No improvement
Let’s examine the sentence: “Would you mind closing the window?” Here, ‘mind’ is acting as a transitive verb. A transitive verb requires an object to complete its sense. This object can be provided by a noun, pronoun or gerund. So, the gerund ‘closing’ is an object for ‘mind.’ Therefore, it is correctly used in the sentence and the sentence requires no improvement. So, Option D is the correct answer.