How to know that a bond is ionic or covelent by just looking at it ?

Ionic bonds are made up of a metal and a nonmetal elements while covalent bonds are are made up of two metal. So, you can just look at the periodic table and identify which compounds have two metals and which have one metal and one nonmetal. Also, those compounds with high electronegativity difference are very likely to be ionic while the opposite is covalent. HOWEVER, some compounds can be both covalent and ionic. For instance, if HCI gas is at higher temperatures, then it is ionic while it would be covalent at room temp. 

Hope I've helped. 

            - Dotz

The correct answer is option (A).

Explanation:

According to Law of Conservation of Mass:

The mass can neither be created nor be destroyed.

In a balanced chemical reaction the total mass of the reactants are equal to the total mass of the products.

So, the mass gain by the nail after rusting was equal to the mass of iron and oxygen which have reacted together to give that rust.

Hence, the correct answer is option (A).