If the distance between a crest and its consecutive trough for a sound is L, then its wavelength is given by:
$\begin{align}
  & A.\text{ }\dfrac{L}{2} \\
 & B.\text{ L} \\
 & C.\text{ 4L} \\
 & D.\text{ 2L} \\
\end{align}$

Hint: Recall the definition of wavelength and thereafter compare the value of given quantity with the wavelength. In this way students will get the required relation. By drawing a rough diagram will get a better picture.

Complete step by step answer:
The minimum distance after which the state of disturbance is repeated in wave motion is called the wavelength of the wave.

Here it is given that the distance between a crest and its consecutive trough is L.
But by the definition of wavelength we know that the wavelength is the distance between two consecutive crests or trough.
According to the information given in the question, L is equal to the half of the wavelength.
So, the wavelength will be 2L.
Hence, the correct option is D, i.e., 2L.

Additional Information:
By definition, the distance between two corresponding crests or trough is known as Wavelength. Or we can say that the distance between two consecutive waves is also known as Wavelength. It is represented by Greek letter λ. Frequency and wavelength is inversely proportional to each other, so if the wavelength is shorter, then the frequency of the signal is higher.

A sound wave is the pattern caused by the movement of energy travelling in any medium, like air, water, etc. There are many different types of waves like electromagnetic waves, water waves, etc.

Note: Students should keep in mind some basic definitions related to waves such as frequency, wavelength, amplitude, time period etc. So that they can answer this type of question very easily even without using pen and paper i.e., by mental calculation. Sometimes get confused between wavelength definition, they may think its distance between crest and trough.