I believe this would be C:
The way I like to think of it is that a direct cause is sorta like something happening on purpose or as expected and an indirect is more unexpected. An easy example might be: Direct: The floor by the elevator was wet so I slipped and fell. Indirect: The floor by the elevator was wet so I took the stairs and fell down them. Hopefully that clarifies the difference.
In the case of C: Jack really slew the giant b/c the giant was after him, but the way this reads he slew the giant b/c of the magic beans, which is really just how he ended up there... kinda like taking the stairs to avoid the elevator. Make sense?
