Magnesium oxide is a binary ionic compound from its formula mgo how do you know that mg is the metal

Answer is: because it is first.
Oxide is a chemical compound that contains at least one oxygen atom and one other element. In oxide oxidation number of oxygen is -2. So in magnesium oxide oxidation number of magnesium is +2 (metal form cation).
When writing molecular formulas, metal is written first (because positive oxidation number).

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My answer is opptionc, just check with anyone else