Answer: (3)
Since (A) is the intermediate reactive substance whose concentration is determined from the equilibrium step.
Let us consider the slow step of the reaction
A + B2 → AB + B (slow)
r = k[A][B2]…………….(1)
From the equilibrium step
A2⇌ A + A (fast)
keq = [A]2/[A2]
∴ [A] = keq[A2]1/2
Substitute the value of [A] in equation (1)
r = k[ keq[A2]1/2][B2]
r = k. keq1/2.[A2]1/2. B2
Thus, the order of the reaction is
⇒ 1/2 +1
⇒ 3/2
⇒ 1.5