What is the difference between a platform and a plank?

The platform is a series of statements expressing the party's principles, beliefs, and positions on election issues. The plank is each individual part of the platform. That would be the correct answer in a political arena.

As far as the sport of diving goes, a platform refers to a solid, elevated surface with a foundation underneath it that a person dives from...a plank is a rectangle shaped board (usually spring) that extends out over the water that a person dives from (or walks off as in the case of most pirate ships.)