The pair of compounds that belongs to the same homologous series is

Analyze the question: A homologous series is a family of organic compounds with the same general formula and the same functional group. Successive members differ by a CH₂ unit.

Concept: general formulas

• Alkanes: CₙH₂ₙ₊₂
• Alkenes: CₙH₂ₙ
• Alkynes: CₙH₂ₙ₋₂

Check each option:

• A. C₂H₆ and C₄H₁₀ • C₂H₆: n = 2, so 2n+2 = 6 ✔ (alkane, ethane) • C₄H₁₀: n = 4, so 2n+2 = 10 ✔ (alkane, butane) • Both are alkanes and differ by 2 CH₂ units. They are in the same homologous series.

• B. C₃H₈ and C₃H₆ • C₃H₈ is an alkane (propane). C₃H₆ is an alkene (propene). Different series. ✘

• C. C₂H₄ and C₄H₁₀ • C₂H₄ is an alkene (ethene). C₄H₁₀ is an alkane (butane). ✘

• D. C₄H₁₀ and C₅H₁₀ • C₄H₁₀ is an alkane (butane). C₅H₁₀ is an alkene (pentene) or a cycloalkane. ✘

Answer: Option A.

Tip: Quickly test each formula against CₙH₂ₙ₊₂ (alkane), CₙH₂ₙ (alkene) and CₙH₂ₙ₋₂ (alkyne). If two compounds in a pair fit the same formula, they belong to the same homologous series. If the carbon count is the same but the hydrogen count differs (as in B), they are in different series.