Observe the following reaction sequence:

C₄H₈ (A) → HCl → C₄H₉Cl (B) → NH₃ → C₄H₉NH₂ (C) (Optically active)

C₄H₉NH₂ → NaNO₂ + HCl → C₄H₉OH

Which of the following is correct structure of A, B and C?

The correct option is Option 4 (represented by the structures in the fourth image).

Step-by-Step Explanation:

1. Identification of Compound A:
The starting compound A has the molecular formula $C_4{H}_8$, which indicates it is an alkene. Based on the options, A can be either 1-butene or 2-butene.

2. Reaction of A with HCl to form B:
When 2-butene (${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3$) reacts with $\mathrm{HCl}$, it undergoes electrophilic addition to yield 2-chlorobutane (B):
${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3+\mathrm{HCl}\to {\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{Cl}\right)-{\mathrm{CH}}_3$

3. Reaction of B with NH₃ to form C:
When 2-chlorobutane (B) reacts with ammonia (${\mathrm{NH}}_3$), a nucleophilic substitution reaction occurs, replacing the chlorine atom with an amino group to yield 2-aminobutane (C):
${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{Cl}\right)-{\mathrm{CH}}_3+{\mathrm{NH}}_3\to {\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CH}\left({\mathrm{NH}}_2\right)-{\mathrm{CH}}_3+\mathrm{HCl}$

4. Optical Activity of Compound C:
The product 2-aminobutane (sec-butylamine) contains a chiral carbon atom at C-2, which is bonded to four different groups: a hydrogen atom ($-H$), a methyl group ($-{\mathrm{CH}}_3$), an ethyl group ($-{\mathrm{CH}}_2{\mathrm{CH}}_3$), and an amino group ($-{\mathrm{NH}}_2$). Because of this asymmetric carbon, 2-aminobutane is optically active, which matches the problem description.

5. Reaction of C with NaNO₂ + HCl:
The reaction of primary aliphatic amines with nitrous acid (${\mathrm{NaNO}}_2+\mathrm{HCl}$) forms an unstable diazonium salt, which undergoes hydrolysis to yield the corresponding alcohol, 2-butanol ($C_4{H}_9\mathrm{OH}$).

6. Analyzing the Provided Options:
- Option 1 (image_0): Shows A as 1-butene, B as 1-chlorobutane, and C as 1-aminobutane (which is optically inactive).
- Option 2 (image_1): Shows C as 2-butanol instead of the amine.
- Option 3 (image_2): Shows C as 2-butanol instead of the amine.
- Option 4 (image_3): Shows A as 2-butene, B as 2-chlorobutane, and C as 2-aminobutane. This is the correct structural match.