To determine the formula of the compound iron (III) phosphide, we'll need to consider the charges and how they balance out between the iron (III) ion and the phosphide ion.
1. Iron (III), represented as \( \text{Fe}^{3+} \), carries a charge of +3.
2. Phosphide, represented as \( \text{P}^{3-} \), carries a charge of -3.
These charges must balance each other for the compound to be neutral. Since the charges are equal in magnitude but opposite in sign (+3 for iron and -3 for phosphide), one ion of each is necessary to balance the charges.
The resulting chemical formula, in this case, would be:
[tex]\[ \text{Fe}^{3+} + \text{P}^{3-} = \text{FeP} \][/tex]
So, the correct formula of the compound iron (III) phosphide is \( \text{FeP} \).
Thus, the answer is:
D. [tex]\( \text{FeP} \)[/tex]
