$SN^2$ mechanism occurs as
In $SN^2$ reaction , in the transition state there will be five groups attached to the carbon atom at which reaction occurs.
Thus there will be crowing in the transition state, and the bulkier the group, The more reaction will be hindered sterically.
Hence $SN^2$ reaction is favoured by small groups on the carbon atoms attached to halogens.
So, the decreasing order of reactivity of halide is
$RCH_2 X > R_2 CHX > R_3CX$
in the order f primary, secondary and tertiary.
Hence c is the correct answer.