(c) For an $n$th order reaction $t_{1/2} \propto \frac{1}{a^{n - 1}}$ For 1^{st} order reaction ,$t_{1/2} \propto \frac{1}{a^{1 - 1}} \propto a^{0}$
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.