The writer says every task took about five times its normal duration because the air at that height was too thin to breathe comfortably. So the work took long because there was not enough air, option (b). Tiredness, cold and darkness are not given as the reason: the passage describes the tent as not too cold, and it never mentions darkness.
Why each option is right or wrong
(a) Wrong. Tiredness is not given as the reason for the delay.
(b) Right. Tasks took five times as long because there was too little air at that height.
(c) Wrong. Cold is not the reason; the tent was not too cold.
(d) Wrong. Darkness is never mentioned.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
