Question

which alky1 halide follows only SN2 hydrolysis mechanism

Answer 1

1degree alkyl halide will prefer only sn2 mechanism

Answer 2

1° alkyl halide follows only $S_N2$ hydrolysis mechanism. 
          there are two reason, 1° alkyl halide follow $S_N2$ hydrolysis reaction. 
   (1) 1° substrate have little steric hindrance or steric crowding  to nucleophilic attack :- as you know that more the number of alkyl groups are added to the α-Carbon atom, the substrate becomes less capable to $S_N2$ attack. here 1° alkyl halide has only one alkyl group. thats why it has capable to $S_N2$ reaction.

(2) 1° Carbocation is relatively unstable :- increasing the number of alkyl group , increases the stability of carbocation. here 1° alkyl group  has only one alkyl group . so, comparetively it is less stable. hence, it is hardly to form 1° carbocation by $S_N1$ reaction. instead  it will prefer $S_N2$ reaction and avoid the formation of unstable carbocation.