Pure substances are characterized by:
(A) variable melting points.
(B) non-uniform composition.
(C) variable properties.
(D) fixed boiling points.

Hint: A pure substance is a single substance. It has uniform composition throughout. Thus, the composition of a pure substance is definite and its composition is same in different parts of the sample.

Complete answer:
You can classify the substances in pure substances and mixtures. You can further classify mixtures into homogeneous mixtures and heterogeneous mixtures. Examples of homogeneous mixtures include sugar dissolved in water and salt dissolved in water. Examples of heterogeneous mixtures include the mixture of iron powder and sulphur powder.
In pure substances, atoms of only one type are present.
Hydrogen, oxygen, nitrogen etc. are some examples of pure substances. Water is another example of pure substance. In water, two elements hydrogen and oxygen are present. These two elements combine in a fixed ratio to form water. The physical and chemical properties of water are different from the physical and chemical properties of hydrogen and oxygen.
The physical and chemical properties of pure substances are fixed. Thus, pure substances have fixed melting points, boiling points, solubility and density.
Thus, pure substances are characterized by fixed boiling points.
Variable melting points, non-uniform composition and variable properties are properties of mixtures.

Hence, the correct option is the option (D).

Note: There are two types of pure substances, elements and compounds. Elements have atoms of only one type. Compounds have atoms of two or more types, but the relative ratios of these atoms are fixed.