1) 3mgcl2
coefficient:
subscript for mg:
subscript for ci:

In "3 MgCl₂", the coefficient is 3, the subscript for Mg is 1 and the subscript for Cl is 2.

Let's consider the expression 3 MgCl₂.

In "3 MgCl₂", the coefficient is 3, the subscript for Mg is 1 and the subscript for Cl is 2.

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Coefficient = 3

Subscript for Mg = 1

Subscript for Cl = 2

Explanation:

Coefficient:

A coefficient is number before the chemical formula which tells the number of molecules take part in chemical reaction.

For example:

2H₂ + O₂    →  2H₂O

In this reaction 2 moles of hydrogen react with one mole of oxygen and produce 2 mole of water. The number 2, 1 and 2 are the coefficient.

In given formula 3MgCl₂ the number before the formula which is three is coefficient.

While the number written as subscript tell about the number of atoms of elements. e.g.in MgCl₂ two atoms of chlorine and one atom of magnesium are present in given formula. If only one atom of particular element is present then it is not written as subscript.

i think it's letter a.