In UML modeling, a constraint is defined as:
A. a condition that must be satisfied when it is evaluated
A constraint is a semantic condition or restriction expressed in natural language text or a machine-readable language for expressing constraints, such as OCL (Object ConstraintLanguage). Constraints specify invariants that must hold for the system being modeled at all times. This means that whenever the constraint is evaluated, the condition it expresses must be satisfied (UML 2.5 specification, section 7.9).
The other options do not accurately define what a constraint is in the context of UML:
B. A condition that should be met depending on system operation – This is not precise as constraints are not optional and do not depend on system operation; they are always applicable.
C. A condition that causes the state of the objects to change over time – This describes a side effect, which is not the purpose of a constraint. A constraint is a condition that must always be met, not something that induces change.
D. A condition that constrains what can or cannot be put in a class diagram – This is too broad and imprecise. Constraints apply to elements within the class diagram and are not about the content of the diagram itself.
