The order with respect to a reactant is the power to which its concentration is raised in the experimentally determined rate law, and it can be found from how the rate responds to concentration changes. If rate ∝ [A]^x, then doubling [A] multiplies the rate by 2^x. Here the rate becomes four times larger, so 2^x = 4, which gives x = 2. The reaction is therefore second order with respect to that reactant. The zero-order option would leave the rate unchanged on doubling. First order would only double the rate. Third order would multiply the rate by eight. Order is an empirical quantity that need not match the stoichiometric coefficient, a key NCERT distinction between order and molecularity. Plausibility check: a fourfold rate increase corresponds exactly to squaring the doubled concentration, so second order is the only consistent value.