A substance composed of two or more elements in a fixed, definite proportion is

a. an alloy.

b. a compound.

c. a heterogeneous mixture.

d. a solution.

e. a homogeneous mixture.

B

Explanation:

A is wrong

An alloy is a mixture of two or more metals.

C is wrong

The above definition does not work for a heterogeneous mixture. If they were a mixture, they would not have a definite proportion

D is wrong

A solution is formed when a solute is dissolved in a solvent. There is no case of dissolving anything in the definition so it cannot be

E is wrong

Just like C, this definition does not work

B is correct

A compound contains two or more elements which are in a fixed and definite proportion