Answer:
Oxidizing element: Mg
Explanation:
Reaction of magnesium and oxygen can be written as:

while writting chemical reaction between two atom or element then write their natural form i.e. how that atom or element exist in the nature.
eg. reaction of any element with Oxygen atom then we can not write directly O because oxygen exist in the nature in form of 
finding oxidation number each side:
Reactant side:
Mg=0;
O=0;
product side:
Mg=+2;
O=-2;
from above:
oxidation number of Mg is increasing hence this is oxidizing element
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