The student has learnt the answer by heart.

The answer was learnt by the student by heart.
The answer is learnt by heart by the student.
The answer by the student was learnt by heart.
The answer has been learnt by heart by the student.

The correct answer is: D. The answer has been learnt by heart by the student.

The other options are incorrect because they do not use the correct tense. The sentence is in the present perfect tense, so the answer must also be in the present perfect tense.

  • A. The answer was learnt by the student by heart. This sentence is in the simple past tense, which is incorrect.
  • B. The answer is learnt by heart by the student. This sentence is in the simple present tense, which is also incorrect.
  • C. The answer by the student was learnt by heart. This sentence is grammatically correct, but it is not the most concise way to say the sentence. The phrase “by the student” is unnecessary, as the word “student” is already the subject of the sentence.

Therefore, the correct answer is D. The answer has been learnt by heart by the student. This sentence is in the present perfect tense, which is correct. It is also concise and easy to understand.