A function $f$ satisfying $f \circ f \circ f(x)=x$ for all $x \in \mathbf{R}$ is

• A

  onto but not one-one.
• B

  neither one-one nor onto.
• Correct answer
  C

  one-one and onto.
• D

  one-one but not onto.

Hence, the correct answer is option: 'C'.