In the following given reaction 'A' is

Step 1: Identify the starting compound

The structure shows a cyclobutane ring with an exocyclic double bond and a methyl group:

    CH3
     |
     C==CH2
    / \
   |   |
    \_/

This is methylenecyclobutane with structure: cyclobutane with $\ce{=CH2}$ group.

Step 2: Understand HBr addition

HBr adds to alkenes following Markovnikov's rule: H goes to the carbon with more H atoms, Br goes to the more substituted carbon.

But with strained rings like cyclobutane, ring expansion can occur to relieve ring strain.

Step 3: Ring expansion mechanism

When HBr adds:

1. Protonation of the double bond forms a carbocation
2. The cyclobutane ring (4-membered, highly strained) undergoes ring expansion to form a more stable 5-membered ring (cyclopentane)
3. Br⁻ attacks the carbocation

The mechanism: $\ce{methylenecyclobutane + H+ -> carbocation (with ring expansion) -> cyclopentane derivative}$

Step 4: Determine the product

After ring expansion and Br addition, the product is:

• Cyclopentane ring (5-membered, more stable than 4-membered)
• $\ce{Br}$ and $\ce{CH3}$ on adjacent carbons

Looking at option (d): This describes a cyclopentane with substituents, which matches ring expansion.

Answer: (d)

Key Points:

• Small rings (3, 4-membered) are strained and undergo ring expansion
• Ring expansion: 4-membered → 5-membered (more stable)
• Carbocation rearrangements favor more stable products