The correct formula for sodium phosphide is \( \text{Na}_3\text{P} \).
In this formula:
- Sodium (Na) has a charge of +1.
- Phosphide (P) has a charge of -3.
To balance the charges, three sodium ions (Na\(^+\)) are needed for each phosphide ion (P\(^{3-}\)), resulting in the formula \( \text{Na}_3\text{P} \).