Which statement is not true about a wave in a plucked string.

The question asks which statement is not true about a wave in a plucked string.

Let's analyze each option:

• A) It has a crest: A wave on a plucked string is a transverse wave, which is characterized by crests (points of maximum displacement above the equilibrium position) and troughs (points of maximum displacement below the equilibrium position). So, this statement is true.
• B) It is transverse: In a transverse wave, the particles of the medium oscillate perpendicular to the direction of wave propagation. When a string is plucked, the string particles move up and down, while the wave travels along the length of the string. This is the definition of a transverse wave. So, this statement is true.
• C) It is longitudinal: In a longitudinal wave, the particles of the medium oscillate parallel to the direction of wave propagation (e.g., sound waves). Waves on a plucked string do not exhibit this type of particle motion. So, this statement is false.
• D) It is mechanical: A mechanical wave requires a medium to propagate and involves the transfer of energy through the vibration of particles in that medium. A wave on a plucked string requires the string itself as a medium. So, this statement is true.

Since the question asks for the statement that is not true, the correct option is C.

The final answer is $\boxed{\text{C) It is longitudinal}}$.