The correct answer is: A. Knows his subject thoroughly well.
A teacher is successful only if he knows his subject thoroughly well. This is because a teacher who knows his subject well can effectively communicate the material to his students and help them learn. A teacher who does not know his subject well will not be able to do this effectively, and his students will not learn as much.
The other options are not as important as knowing the subject well. A teacher who produces cent per cent results may be doing a good job, but this is not the only measure of success. A teacher who is approachable may be popular with his students, but this is not the only measure of success either. A teacher who publishes papers in journals of repute may be a good researcher, but this is not the only measure of success.
The most important thing for a teacher is to know his subject well. If he does this, he will be able to effectively communicate the material to his students and help them learn.