Which formula identifies a single substance?

"A formula identifies single substance" means that the formula corresponds to "one substance" only.  Otherwise, the formula corresponds to two or more isomers.

A. CH₃C*HClCH₂CHO has one chiral center (shown by "*").  The formula corresponds to two stereoisomers (two enantiomers).

B. Each bonding C atom of the C=C double bond in CH₃CH=CHCH₃ is also bonded to a -CH₃ group and an -H atom.  The formula corresponds to two stereoisomers (two geometrical isomers).

C. For CH₂ClCH₂CHCl₂ does not have branch because there are only 3 carbon atoms, and thus there is no structural isomers.  Besides, there is no chiral center or C=C double bond, and thus there is no stereoisomers.  Hence, the formula corresponds to one single compound only.

D. C₄H₁₀ has two structural isomers, namely butane and 2-methylpropane.

The answer: C. CH₂ClCH₂CHCl₂