Which of the following is the correct order of decreasing $$SN^2$$ reactivity?

$$S_N2$$ reactions occur through a single transition state in which the nucleophile attacks the substrate carbon from the backside while the leaving group departs.
Hence, any crowding (steric hindrance) at the electrophilic carbon slows the reaction.

Order of steric hindrance around the carbon bearing the leaving group is:

Primary halide $$\big(RCH_2X\big)$$ < Secondary halide $$\big(R_2CHX\big)$$ < Tertiary halide $$\big(R_3CX\big)$$.

Because the rate of an $$S_N2$$ reaction is inversely related to steric hindrance, the reactivity order is exactly the reverse of the steric order:

$$RCH_2X \; \gt \; R_2CHX \; \gt \; R_3CX$$.

This sequence matches Option B.

Therefore, the correct order of decreasing $$S_N2$$ reactivity is:

Option B which is: $$RCH_2 X \gt R_2 CHX \gt R_3 CX$$