Both BitcoinApplication and OptionsModel classes are derived from the QObject class, therefore a parent-child relation could be established to manage the lifetime of an OptionsModel object:
https://github.com/bitcoin/bitcoin/blob/5236b2e267a58870239673c7ec85e5df0cb8fc8e/src/qt/optionsmodel.cpp#L29-L30
This PR does not change behavior.