The quantity k in a rate law expression _____

is independent of concentration

The correct answer is Option (4) → is independent of concentration

In a rate law expression, the quantity k is the rate constant.

• Independent of temperature --  incorrect
  → k depends strongly on temperature (Arrhenius equation).
• Called the Arrhenius constant -- incorrect
  → The Arrhenius constant is A, not k.
• Dimensionless -- incorrect
  → Units of k depend on the order of the reaction.
• Independent of concentration  -- correct
  → For a given reaction at fixed temperature, k does not depend on reactant concentrations.