Which of the following alkenes exhibits geometrical (cis–trans) isomerism?

**Test.** For cis–trans isomerism each sp²-carbon of the C=C bond must carry two *different* groups. If either carbon has two of the same substituent, the two configurations are identical. - **A. 2-methylpropene** $(CH_3)_2C=CH_2$ — left carbon has two CH₃ groups → no. - **B. Propene** $CH_3CH=CH_2$ — the CH₂ end has two H → no. - **C. But-2-ene** $CH_3CH=CHCH_3$ — each sp²-C carries one CH₃ and one H → **yes**, gives cis (Z) and trans (E). - **D. Pent-1-ene** $CH_2=CHCH_2CH_2CH_3$ — terminal C has two H → no. The rule of thumb: terminal alkenes never show cis–trans internal alkenes do *only if* the two substituents on each end differ.