Answer: The option that best explains this situation is that Ben found his work to be routine and monotonous.
Of all the options listed above, the option that best explains this situation is the fact that Ben had found his work to be routine and monotonous. The question didn't say Ben felt the hike was monotonous, Ben didn't lack ambition, he wasn't motivated by the prospect of extrinsic rewards, neither was he gunning for a position at the competing firm all along.
The question subtly suggest that his work is routine and monotonous in "Ben has been working as a process executive at an accounting firm for the past two years".