Final answer:
The incorrect statement is option (a), which incorrectly identifies the phosphate ion as a positive trivalent ion. The phosphate ion is actually a polyatomic ion with a trivalent negative charge.
Step-by-step explanation:
The student is asked to choose the incorrect statement from given options that relate to the charges and compositions of ions. In this case, the incorrect statement is option (a) which states that the phosphate ion is a positive trivalent ion. In reality, the phosphate ion is represented with the formula PO43- and is a polyatomic ion with a negative trivalent (3-) charge, not positive.
Calcium ion, as stated in option (b) should actually be a divalent positive ion (Ca2+), not trivalent. Phosphorus does exist as a polyatomic molecule, typically in the form of various polyatomic ions like phosphate. Finally, the oxide ion is in fact a divalent negative ion (O2-), rather than a divalent positive ion as incorrectly stated in option (d).