The correct answer is C. Her kind deed will be her reward.
A. Her kind deed will reward her is incorrect because the subject and verb do not agree. The subject is “her kind deed,” which is singular, but the verb is “will reward,” which is plural.
B. Her kind deed will have rewarded her is incorrect because the tense of the verb does not agree with the tense of the sentence. The sentence is in the future tense, but the verb “will have rewarded” is in the past perfect tense.
D. Her kind deed will have been her reward is incorrect because the tense of the verb does not agree with the tense of the sentence. The sentence is in the future tense, but the verb “will have been” is in the past perfect tense.
The sentence “She will be rewarded for her kind deed” means that she will receive something in return for her kind deed. The reward could be anything, such as praise, gratitude, or a material gift. The kind deed itself is the thing that will reward her.