Which sentence uses the conditional form of the verb send?

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A. I am sending a letter to our grandparents.
B. I could send a letter to our grandparents.
C. I suggest you send a letter to our grandparents.
D. Send a letter to our grandparents.

B. I could send a letter to our grandparents.

The sentence that uses the conditional form of the verb send is option C: "I suggest you send a letter to our grandparents."