In the below diagram half the wavelength in the curve is:

The wavelength is the distance between two points in a wave and is denoted by the symbol lambda $\lambda$,as we know that sounds are represented in the form of waves in the air, they are lengths of two particular similar points, so the sounds are represented with wavelengths, and the frequency is the number of waves produced in a specific time.

Each point is represented by the points in the above image.

According to the wavelength, the points B and F are referred to as the crests, while the points D and H are referred to as the troughs. The crest is the largest or peak upward displacement of the wave, and the trough is the least downward displacement.

The wavelengths are the distances between the points B and F, D and H, which is the distance between any two crests and two troughs.

As a result, the half wavelength is defined as half the distance between B and D or D and F.

The distance between B and D is the half wavelength, according to the selections.

Hence, In the given diagram half the wavelength in the curve is: BD.