[poshakpathak]

Structural isomerism and stereoisomerism should be considered when answering this question.
Which formula identifies a single substance?
A CH₃CHClCH₂CHO
B CH₃CHCHCH₃
C CH₂ClCH₂CHCl ₂
D C₄H₁₀

Also, how do you recognise if a compound is single?

[AWK]

You have a carbon atom with 4 different substituents - what does this mean?
You have a compound with a double bond - what does that mean?
You have a general hydrocarbon formula - can it have isomers?

[hollytara]

A formula represents a single compound if there are no places where the formula can be drawn in different ways. 

As AWK points out - a C with 4 different substituents has more than one way to be drawn, a double bond where each Carbon of the double bond has two different substituents has more than one way to be drawn, some cyclic structures may have more than one way to be drawn.  In those cases, the structure is not sufficiently specific to indicate a single compound - it could be one of several options.