The approach described in option B aligns with the concept of template classifiers in UML, which allows for the creation of parameterized elements that can be instantiated with different arguments. By modelingVehicle as a template classifier with a parameter namedThrustor, other modelers can instantiate this template with their chosen propulsion system, as long as it is a type that specializes thePropulsionSystemabstract class. This method provides the flexibility needed for modelers to tailor theVehicleclass to different propulsion systems while enforcing that only derivatives ofPropulsionSystemcan be used as the actual parameter forThrustor.
This tactic of using template classifiers and class parameters is an application of UML's syntax and semantics for creating modular and customizable models. This practice is detailed in the UML 2.x Superstructure Specification, particularly in the sections describing template classifiers and their parameters.