The passage is based on the idea that

• (a) man should not work hard — Too extreme. Such passages usually criticize excessive labour, not hard work itself. Verdict: Incorrect.
• (b) the great evil of our age is overstrain — This captures the central idea directly: the passage points to overstrain/overwork as the main problem of the age. Verdict: Correct.
• (c) man cannot think well — This is only a consequence of overstrain, not the main thesis. Verdict: Incorrect.
• (d) man cannot care for his spiritual welfare — Also a secondary effect, not the core idea. Verdict: Incorrect.

Therefore, the correct answer is (b).