In which reaction does the oxidation number of oxygen increase (i.e., becomes less negative or more positive)?

• A. $2H_2{O}_2\to 2H_{2}O+O_2$
• B. $C_3{H}_8+5O_2\to 3{\mathrm{CO}}_2+4H_{2}O$
• C. $2{\mathrm{Mg}}_{\mathrm{}}+O_2\to 2\mathrm{MgO}$
• D. $2H_{2}O\to 2H_2+O_2$

Answer: (D)