About D Rank Missions...

D Rank Missions are special missions were you get special rewards. Some event missions can appear here.

D Rank Missions that involves Elements are repeatable, and are never updated, and the reward is just the Element. No EXP.
D Rank Missions that involves events are limited missions, some might be repeatable, others not, but they will not stay forever in the game, so it's recommendable to do these missions because they bring great rewards.
D Rank Missions may evolve Digivolutions too. For example, a quest to make EXVeemon evolve to Paildramon. They are never updated and are repeatable of course.
These missions may involve winning DATA from unique Digimons.

Have fun with the D missions, and good luck.