Correct answer is (a) 51.5 m
Easiest explanation: If radius of tube is R then, arc l is equal to Rα where α is taken in seconds. 1 radian is approximately equal to the 206,265 seconds of arc. 10 sec. = 0.0000485 radian. Tilt = 2.5 mm/R. By this, R = 51.5 m approximately.