Stationary Waves & Vibrating Strings question from NEET 2015 (AIPMT)

Answer: (b)

1. Consecutive resonant frequencies of a string differ by the fundamental: $f_0 = f_{p+1} - f_p$.
2. $f_0 = 420 - 315 = 105\text{ Hz}$ (check: $315 = 3 \times 105$, $420 = 4 \times 105$).
3. So the answer is (b) $105\text{ Hz}$.