Answer:
The atoms are arranged differently in each molecule.
Step-by-step explanation:
In H₂O, the bonds are arranged in the order H-O-H. In H₂O₂, they are in the order H-O-O-H.
The O-O bond in hydrogen peroxide is so weak that H₂O₂ tends to explode.
Water has only strong O-H bonds, so it is quite stable.
B is <em>wrong</em>. The molecules have different formulas, so they are different compounds.
C is <em>wrong</em>. The atoms are the same, but their different arrangement causes different properties.
D is <em>wrong</em>. A molecule of water has <em>fewer</em> atoms than one of hydrogen peroxide.