I am playing a ranger and I have both Hobbling Strike and World Serpent's Grasp. Say I use Twin Strike and both attacks hit. On the first hit I choose to forego Hunters Quarry to slow the target and on the second hit I knock the target prone. Can the slowed, prone target still take a crawl action?
They can crawl while slowed, however they will only move 1 square (assuming nothing else is affecting their speed).
Slowed is a condition that changes their speed to 2.[DDI] Crawl is a move action that allows one to move up to half one's speed.[DDI] Since their speed is now 2, crawling would move them up to 1 square.