The correct answer is D. ready to fulfill his promise.
The phrase “as good as his word” means that someone is reliable and does what they say they will do. In this case, the teacher was as good as his word because he was ready to fulfill his promise to help the student.
Option A is incorrect because it means that someone is unable to act. The teacher was able to act on his promise to help the student.
Option B is incorrect because it means that someone is better than expected. The teacher was not better than expected, he simply fulfilled his promise.
Option C is incorrect because it means that someone is highly pretentious. The teacher was not pretentious, he was simply reliable.