What does the string {guarded} mean?

Consider the following diagram:

What does the string {guarded} mean?
A . The execution of op1 is guarded by one or more exceptions which it will handle if they arise.
B . Multiple invocations of op1 that overlap in time may occur to one instance, but only the first one will get handled.
C . Multiple invocations of op1 that overlap in time may occur to one instance, but only one is allowed to commence.
D . The execution of op1 Is guarded by one or more exceptions. Behaviors invoking it are required to handle these exceptions

Answer: C

Explanation:

In UML, when an operation is marked with the term {guarded}, it means that the operation is subject to a concurrency constraint. It specifies that the operation cannot have overlapping executions; in other words, if the operation is already in execution, a new attempt to start it will have to wait until the current one completes. This prevents race conditions and ensures thread safety for that operation when dealing with concurrent accesses in multi-threaded environments. This interpretation is based on the UML 2.x Superstructure Specification, which describes how operations can be marked with constraints to define their behavioral features in terms of concurrency.