Which of the listed chemical elements forms only one simple substance? a) hydrogen b) carbon c) oxygen d) phosphorus

First, what is a simple substance? A simple substance is a substance consisting of molecules of the same atom.

Consider the answer options:

Carbon (C) forms about 9 allotropic modifications (substances with the same formula, but different in structure), for example graphite and diamond.
Phosphorus (P) also forms allotropic modifications such as white phosphorus and red phosphorus.
Oxygen (O) also forms allotropic modifications such as oxygen and ozone.
Hydrogen (H), on the other hand, does not have any allotropic modification.