Which of the following is a substance formed by the chemical joining of two or more elements in definite amounts?

A. enzyme

B. nucleus

C. isotope

D. compound

A substance formed by the chemical joining of two or more elements in definite amounts is a d. compound.

The other answer options are not correct based on the following explanations. An enzyme is a biochemical catalyst made of proteins. A nucleus is a component of most cells where the cell's genetic material is stored and where most of the activities within the cell are regulated. A nucleus is also the location in an atom that contains protons and neutrons. An isotope is a form of an element where the number of neutrons in the nucleus of the atom differs from another atom of the same element (has the same number of protons). Therefore, none of the other options can be correct.