The IUPAC name of CH₃CH=CHCH₃ is:

The molecule CH₃CH=CHCH₃ contains four carbon atoms and one C=C double bond, so its parent name is **butene**. 1. **Locant assignment**: Number the chain to give the double bond the lowest possible locant. Starting from the left carbon: - C1: CH₃ - C2: CH= - C3: =CH - C4: CH₃ The double bond occurs between C2 and C3, so the locant is **2**. 2. **IUPAC format**: The correct name is written as **but‑2‑ene** (parent name + locant + bond‑type suffix). Therefore the correct choice is **(b) But‑2‑ene**. **Why the other options are wrong:** - (a) *But‑1‑ene* would imply the double bond is between C1 and C2 (CH₂=CHCH₂CH₃), which does not match the structure. - (c) *1‑Butene* and (d) *2‑Butene* are not the standard IUPAC format; the correct format requires a hyphen: **but‑2‑ene**.