It’s D because none of the other options make sense. A wouldn’t make sense as it’s saying that him teaching great lessons isn’t among the reasons of this award.
B doesn’t make sense because “fortunately” is often used for compare and contrast and stuff like.
C doesn’t make sense because meanwhile is a time transition. This would mean that while he is a great teacher who deserves this award, he is currently doing something that is not deserving of this reward.
D is correct as its saying that the most important reason is his skill for learning.
