Answer:
a) can have varying properties
Step-by-step explanation:
Mixture:
A mixture is substance made up of two or more kind of substances. In mixture the substances are physically combine with each other and can not react chemically. That's why every substance retain their properties in mixture and thus properties of mixtures can vary according to the addition of substances.
All other options are incorrect because,
b)has a fixed composition
Mixture can not have fixed proportion. The amount of substances that form mixture can be vary.
c) contains no pure substances
Mixture consist of pure substances because all substituents are combine physically and when separated they remain same as they were.