The most reactive halide towards \(S_N1\) reaction is

The correct answer is option 1.

The intermediate in an \(S_N1\) reaction is a carbocation. The stability of that intermediate determines how favorable that particular reaction is, so a more stable intermediate means a more favorable reaction.

Carbocations are quite unstable on their own, but the inductive effect of nearby \(C-C\) bonds provide some electron density to balance out the positive charge, making the carbocation intermediate more stable. Tertiary carbons have the largest number of adjacent \(C-C\) bonds, the largest inductive effect, the most stable carbocation intermediate, and are thus favored in \(S_N1\).

Secondary alkyl halides are more likely to undergo \(S_N1\) reactions under certain conditions:

Polar protic solvents: \(S_N1\) reactions are favored in polar protic solvents such as water, alcohols, and carboxylic acids. These solvents stabilize the carbocation intermediate formed in the reaction.

Good leaving group: The halogen atom in the alkyl halide should be a good leaving group, such as chloride, bromide, or iodide.

Weak nucleophile: \(S_N1\) reactions favor weak nucleophiles, as the rate-determining step involves the formation of a carbocation intermediate, which is then attacked by the nucleophile.

Secondary alkyl halide: Secondary alkyl halides are more likely to undergo \(S_N1\) reactions compared to primary alkyl halides, as they form more stable carbocation intermediates.

Primary alkyl halides are less likely to undergo \(S_N1\) reactions because the resulting primary carbocation is less stable due to the absence of additional alkyl groups.

Out of the given options, the compounds in option (2), (3) and (4) are all primary alkyl halides and option (1) is a secondary alkyl halide, thus option (1) will be most reactive towards \(S_N1\) reaction.