The black, granular material that fills a dry cell in a common flashlight (between the carbon rod and the zinc shell) is manganese dioxide, MnO2. The oxidation number of manganese in MnO2 is:

a. +2
b. -2
c. +4
d. -4

c. +4

Explanation:

For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.  

Here manganese is having an oxidation state of +4 called as  cation and oxygen forms an anion with oxidation state of -2. Thus they combine and their oxidation states are exchanged to give which written in simplest whole number ratios to give neutral

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