18) B

To solve this question, we need to balance the charges to 0.

A – Wrong
Mg = 2+
2H = 2+
2PO₄ = 6-
2 + 2 – 6 = -2

B – Correct
Mg = 2+
4H = 4+
2PO₄ = 6-
2 + 4 – 6 = 0

C – Wrong
Mg = 2+
3H = 3+
PO₄ = 3-
2 + 3 – 3 = +2

D – Wrong
Mg = 2+
6H = 6+
2PO₄ = 6-
2 + 6 – 6 = +2

E – Wrong
2Mg = 4+
H = 1+
PO₄ = 3-
1 + 4 – 3 = +2

F – Wrong
2Mg = 4+
2H = 2+
PO₄ = 3-
4 + 2 – 3 = +3