k_m value or Michaelis constant is defined as the substrate concentration at which half of the enzyme molecules are forming (ES) complex or concentration of the substrate when the velocity of the enzyme reaction is half the maximum value. The k_m varies from enzyme to enzyme and is used in characterizing the different enzymes. A smaller k_m value indicates greater affinity of the enzyme for its substrate, hence, shows a quicker reaction k_m is a constant characteristic of an enzyme for its conversion of a substrate.

So, the correct answer is option C.