Explanation:

Key Terms:
- Crest: The highest point of a wave.
- Trough: The lowest point of a wave.
- Distance between crest and trough: The vertical distance between a crest and the immediately following trough.

Understanding the Problem:
In this problem, we are given that the distance between a consecutive crest and trough is x. We need to determine the wavelength of the wave.

What is Wavelength?
Wavelength is the distance between two consecutive crests or two consecutive troughs in a wave. It is denoted by the Greek letter lambda (λ).

Solution:
To find the wavelength, we need to understand the relationship between the distance between a crest and trough (x) and the wavelength (λ).

Relationship between x and λ:
- The distance between a crest and trough is equal to half the wavelength.
- In other words, the wavelength is twice the distance between a crest and trough.

Option D Explanation:
According to the given information, the distance between a consecutive crest and trough is x. Therefore, the wavelength can be calculated as follows:

- The distance between a crest and trough is x.
- The wavelength is twice the distance between a crest and trough.
- Therefore, the wavelength is 2x.

Hence, option D, which states that the wavelength is 2x, is the correct answer.

Summary:
The distance between a consecutive crest and trough is x, and the wavelength of the wave is 2x. This relationship can be understood by recognizing that the wavelength is twice the distance between a crest and trough. Therefore, option D is the correct answer.