Answer : The correct formula of the given compound will be,
Explanation :
Iron (III) phosphide is an ionic compound because iron element is a metal and phosphorous element is a non-metal. The bond formed between a metal and a non-metal is always ionic in nature.
The nomenclature of ionic compounds is given by:
1. Positive ion is written first.
2. The negative ion is written next and a suffix is added at the end of the negative ion. The suffix written is '-ide'.
3. In case of transition metals, the oxidation state are written in roman numerals in bracket in-front of positive ions.
The charge on iron is (+3) and the the charge on phosphide is (-3). The charges are balanced.
Hence, the formula of the compound iron (III) phosphide will be