Which of the following method is used to write a chemical formula?

The criss-cross method is used to determine the chemical formula of the given compounds. 

In the criss-cross method, the numerical value of the ion charge of the two atoms is crossed over, which becomes the subscript of the other ion.

For example- the Ion charge of Mg is +2  and the Ion charge of Cl is -1.

Hence, the chemical formula is MgCl₂