Chapter 13 - Vibrations and Waves - Learning Path Questions and Exercises - Multiple Choice Questions - Page 483: 17

(c) one wavelength

In the second harmonic mode, the standing wave in the string exhibits one wavelength. For $n^{th}$ harmonic mode, the wavelength of the standing wave is $L=n(\frac{wavelength}{2})$ where L is the length of the string. So for n=2, we have L=wavelength.