Regioselective HBr addition to multiple unsymmetric double bonds

Steps:

• Examine the structure of X to identify the three C=C bonds and their substitution patterns.
• For each double bond, determine Markovnikov addition orientations, yielding 1 product for symmetric bonds and 2 for unsymmetric ones.
• Since the bonds are isolated (no conjugation effects mentioned), independent additions occur without skeleton change.
• Multiply regioisomers: two bonds with 2 options each, one with 1, totals 4 structural products.

Why B is correct:

• Four products arise from combinatorial regiochemistry across the three double bonds, per the definition of structural isomers as distinct connectivities.

Why the others are wrong:

• A: Underestimates by ignoring multiple unsymmetric bonds.
• C: Overcounts by including stereoisomers, excluded here.
• D: Overcounts by assuming conjugation or rearrangements, not indicated.

Final answer: B