I think 3rd option will be the answer. bcoz 4-chloropent-2-ene is an allylic halide and according to preference allyl halide gives Sn1 reaction faster bcoz the carbonation formed will be more stable than other 3 options. the reactivity order for SN1 reaction is allyl halide > alkyl halide > vinyl halide. hope its helpful.

**Answer:**

To determine which alkyl halide reacts fastest by SN1 mechanism, we need to consider the stability of the carbocation intermediate formed during the reaction. The more stable the carbocation, the faster the reaction will proceed.

**1. 1-chloroethene:**
- This compound does not have any resonance stabilization for the carbocation intermediate.
- The carbocation intermediate formed will be primary, which is less stable than secondary or tertiary carbocations.
- Therefore, 1-chloroethene will react relatively slower by SN1 mechanism.

**2. 2-chloropropene:**
- This compound also does not have any resonance stabilization for the carbocation intermediate.
- The carbocation intermediate formed will be secondary, which is more stable than a primary carbocation.
- Therefore, 2-chloropropene will react faster than 1-chloroethene but slower than compounds with a tertiary carbocation.

**3. 4-chloropent-2-ene:**
- This compound has resonance stabilization for the carbocation intermediate due to the presence of a double bond and a neighboring methyl group.
- The carbocation intermediate formed will be tertiary, which is more stable than secondary or primary carbocations.
- Therefore, 4-chloropent-2-ene will react faster than both 1-chloroethene and 2-chloropropene.

**4. 1-chloropropene:**
- This compound does not have any resonance stabilization for the carbocation intermediate.
- The carbocation intermediate formed will be primary, which is less stable than secondary or tertiary carbocations.
- Therefore, 1-chloropropene will react slower than both 4-chloropent-2-ene and 2-chloropropene.

Therefore, the alkyl halide that reacts fastest by SN1 mechanism is **4-chloropent-2-ene** due to the presence of resonance stabilization and a tertiary carbocation intermediate.