# Which of the following illustrations represents a pure substance? Why?

$\text{Option C}$
$A$ depicts at least two types of substance, a diatomic molecule, ${X}_{2}$, and a heteronuclear diatomic, $A X$; $B$ depicts single atoms, and another heteronuclear diatomic; $D$ depicts the same as $B$.
For $C$, however, only the one type of heteronuclear diatomic is present. Reasonable examples would include $H X$ or $C O$.