Most reactive halide towards S_N1 reaction is

tert-Butyl chloride

The answer is (3) tert-Butyl chloride.

The reactivity of alkyl halides towards SN1 reactions depends on the stability of the carbocation intermediate. The more stable the carbocation intermediate, the more reactive the alkyl halide.

In the case of tert-butyl chloride, the carbocation intermediate is very stable because it is stabilized by three methyl groups. The methyl groups are electron-donating groups, which donate electrons to the carbocation, making it less positive.

The other alkyl halides do not have as stable carbocation intermediates. For example, the carbocation intermediate in n-butyl chloride is not as stable because it is not stabilized by any electron-donating groups.

Therefore, tert-butyl chloride is the most reactive halide towards SN1 reactions.

The other options are incorrect because they are not as reactive as tert-butyl chloride toward S_N1 reactions.

Reactivity of alkyl halides towards S_N1 reaction

The stability of the carbocation in the first step determines the reactivity order. The greater the stability of the carbocation, the greater will be its ease of formation from an alkyl halide and hence faster will be the rate of the reaction. Since the most stable carbocation is at 3°, the tert-alkyl halides will undergo the S_N1 reaction very rapidly. The order of reactivity will be as follows:

tertiary alkyl halide > secondary alkyl halide > primary alkyl halide > methyl halide