Answer : The number placed in front of
should be, three (3).
Explanation :
Balanced chemical reaction : It is defined as the number of atoms of individual elements present on reactant side must be equal to the product side.
The given unbalanced chemical reaction is,

This chemical reaction is an unbalanced reaction because in this reaction, the number of atoms of chloride and sulfate ion are not balanced.
In order to balanced the chemical reaction, the coefficient 3 is put before the
, the coefficient 2 is put before the
and the coefficient 3 is put before the
.
Thus, the balanced chemical reaction will be,

Therefore, the number placed in front of
should be, three (3).