Which of the following sentences correctly punctuates the transitional phrase?
A. For that reason the students with the highest grades in the class are those that study the most.
B. For, that reason, the students with the highest grades in the class are those that study the most.
C. For that, reason, the students with the highest grades in the class are those that study the most.
D. For that reason, the students with the highest grades in the class are those that study the most.