Final answer:
The polyatomic ion (XY) in the compound (XY)Br3 has a +3 charge, to neutralize the -3 charge from the three bromine atoms, making the overall compound neutral.
Step-by-step explanation:
The charge on the polyatomic ion (XY) in the compound (XY)Br3 can be determined by considering the charge of bromine (Br). Bromine, being a halogen, typically has a -1 oxidation state when it forms halide compounds. Since there are three bromine atoms, their combined charge is -3. For the compound to be neutral overall, the polyatomic ion (XY) must have a charge that balances out the -3 charge from the bromine atoms. Thus, the charge on the polyatomic ion (XY) must be +3 to neutralize the charges and make the compound neutral. Therefore, the correct answer is A. +3.