Significant Figures & Order of Magnitude question from JEE Main 2014 (19 Apr, Shift 1)

Answer: (a)

1. Rotation of earth: $1\ \text{day} = 86400\ \text{s} \sim 10^{5}\ \text{s}$, so (a)-(i).
2. Revolution of earth: $1\ \text{year} \approx 3.2 \times 10^{7}\ \text{s} \sim 10^{7}\ \text{s}$, so (b)-(ii).
3. Light wave: $T = \lambda/c \approx 5\times10^{-7}/3\times10^{8} \sim 10^{-15}\ \text{s}$, so (c)-(iii).
4. Sound wave: $f \sim 10^{3}\ \text{Hz}$, $T \sim 10^{-3}\ \text{s}$, so (d)-(iv).
5. So the answer is (a).