For which of the following can the composition vary?
a. both homogeneous and heterogeneous mixtures
b. heterogeneous mixture
c. homogeneous mixture
d. element pure substance

Option (a) is the correct answer.

Explanation:

A homogeneous mixture is defined as a mixture that contains evenly distributed solute particles into the solvent.

For example, NaCl dissolved in water is a homogeneous solution.

On the other hand, a solution in which solute particles are not evenly distributed into the solvent is known as a heterogeneous mixture.

For example, when sand is added to water then it forms a heterogeneous solution.

Thus, we can conclude that for both homogeneous and heterogeneous mixtures the composition vary.