The correct answer is: A. Kannan told Subha that he will return the book after he has read it.
The sentence “I’ll return the book after I have read it” is in the simple future tense. This means that Kannan is making a promise to Subha that he will return the book after he has finished reading it. The simple future tense is used to talk about things that will happen in the future.
The other options are incorrect because they use the past tense or the conditional tense. The past tense is used to talk about things that happened in the past. The conditional tense is used to talk about things that might happen in the future or that are possible but not certain.
In this case, Kannan is making a promise to Subha, so he is using the simple future tense to indicate that he is certain that he will return the book after he has finished reading it.