The oxidation number of oxygen in hydrogen peroxide is:
A. +1.
B. -1.
C. +2.
D. -2.

Hint: In this question we will use the method of finding the oxidation number of an element in a compound. We have to remember that the algebraic sum of the oxidation numbers of all the atoms present in a compound must be equal to zero.

Complete step-by-step answer:
We know that Oxidation number is defined as the charge that an atom of an element has in its ion or appears to have when present in the combined state with other atoms.
 Given compound is hydrogen peroxide. Its chemical formula is \[{H_2}{O_2}\].
Hydrogen peroxide, \[{H_2}{O_2}\] is a chemical compound that's a combination of hydrogen and water. Hydrogen peroxide is a mild antiseptic used on skin to prevent minor cuts, scrapes, and burns from infection. It can also be used as a mouth rinse to help remove mucus or to relieve minor mouth irritation (e.g., gingivitis, due to canker / cold sores). This product functions by releasing oxygen when applied to the area affected. Oxygen release causes foaming which helps remove dead skin and purify the area.
It contains 2 hydrogen and 2 oxygen atoms.
We know that the oxidation number of hydrogen is always +1.
Let the oxidation number of each oxygen (O) atom be $x$.
We know that the algebraic sum of the oxidation numbers of all the atoms present in a compound must be equal to zero.
So,
$
   \Rightarrow 2\left( {{\text{oxidation number of H}}} \right) + 2({\text{oxidation number of O}}) = 0 \\
   \Rightarrow 2( + 1) + 2(x) = 0 \\
   \Rightarrow + 2 + 2x = 0 \\
   \Rightarrow 2x = - 2 \\
$
$
   \Rightarrow x = \dfrac{{ - 2}}{2} \\
   \Rightarrow x = - 1 \\
$
Thus, we have obtained the value of x = -1.
Hence, we can say that the oxidation number of oxygen in hydrogen peroxide is -1.
Therefore, the correct answer is option (B).

Note: Whenever we are asked such types of questions we have to follow some basic steps. First we have to find out the charge on the other elements and the oxidation number of the asked element be x. After that we will multiply the charge of each element with the number of atoms present in the compound. Then we will equate the algebraic sum of the oxidation number of all atoms equal to zero. Then we will find out the oxidation state of the required element.