Choose the sentence with correct use of the comma:
A. Although it was cold they decided to go for a hike.
B. Although, it was cold, they decided to go for a hike.
C. Although it was cold they, decided to go for a hike.
D. Although it was cold, they decided to go for a hike.
The correct form is "Although it was cold, they decided to go for a hike," using a comma to separate clauses.