💡 Detailed Explanation
"Hakusha o kakeru" means to accelerate, speed up, or give impetus to the progress or momentum of something. It is primarily used for abstract concepts, efforts, or momentum. Choice 1 correctly uses it to describe how enthusiasm accelerated the team's morale. The other choices, while using abstract concepts, misapply the idiom: Choice 2 incorrectly applies it to 'atmosphere' where 'enliven' or 'soften' would be more natural; Choice 3 uses it for 'self-growth' where it typically implies an external impetus, making it less natural; and Choice 4 incorrectly applies it to a 'difficult problem' itself, rather than the effort or progress towards solving it.