Identifying the foundational idea regarding overwork and lack of spiritual fulfillment

The passage builds a direct cause-and-effect argument: an "excessive amount of labour" (overstrain) is the root cause preventing man's "spiritual element" from thriving and destroying his capacity for "serious activities." The author frames this relentless overwork as the foundational problem degrading modern human culture. Option (b) captures this diagnosis perfectly. Options (c) and (d) are symptoms/effects of this overstrain, not the foundational idea itself.

Answer: (b).