Which one of the following is a compound?AThe black residue left when sugar is charredBNitrogen gasCAirDCarbon dioxide

Dear student 

Compound is a substance formed from two or more elements chemically bonded in a definite proportion.

In this Carbon di oxide is a compound. 

Because Carbon and oxygen is in definite proportion.

 

ANSWER
Among these only carbon dioxide is a compound.
Compound is a substance formed from two or more elements chemically bonded in a definite proportion.
Example- Carbon dioxide, water, glucose, etc.
In the given options,
A- the black residue that is left after charring sugar is carbon. Carbon is an element in pure state.
B- Nitrogen gas is a molecule as it is composed of only one element ie., Nitrogen so this can't be termed as a compound.
C- Air is a homogeneous mixture of different gases.