Assertion (A) : A Chemical reaction becomes faster at higher temperatures. Reason (R) : At higher temperatures, molecular motion becomes more rapid.

Both the assertion and reason are true, and the reason correctly explains the assertion. Raising temperature increases the rate constant k because, per Arrhenius-type descriptions, the rate depends on both a collision-frequency factor and the Boltzmann fraction of molecules with energy ≥ Ea; as T increases the exponential term grows, speeding reactions [1]. Physically, higher temperature raises the average kinetic energy of molecules so they move faster, increasing collision frequency and the fraction of collisions energetic enough to overcome the activation barrier — exactly the molecular mechanism that makes reactions faster at higher T. Thus the faster molecular motion at higher temperature is the correct explanation for increased reaction rates.

Sources

1. [1] https://www.chem.tamu.edu/rgroup/hughbanks/courses/102/slides/slides17_2.pdf