Answer:
b. Mg (s) + 2HCl (aq) --> MgCl2 (aq) + H2
Explanation:
A subscript applies only to the element it is attached to while coefficients apply to all elements in a compound.
"a" has 1 atom of each element on the left but 2 Cl on the right.
"c" has 2 Mg, 1 H, & 1 Cl on the left but 1 Mg, 2 H, and 2 Cl on the right.
"d" has 2 atoms of each element on the left but only 1 Mg on the right.
"b" is the only answer with equal numbers of atoms on the right and left sides of the equation.
