Final answer:
Messages sent by a process can be either fixed or variable sized, depending on the requirements of the communication protocol or the application's needs.
Step-by-step explanation:
The question pertains to the characteristics of messages sent by a process in a computing context, specifically regarding their size. The correct answer is: c) can be fixed or variable sized. This means that there is no one-size-fits-all rule for the size of messages a process can send. The size of a message may be determined by the specific requirements of the communication protocol or the application's needs. For example, some low-level networking protocols or systems might define a fixed-size message for simplicity and efficiency, while many high-level application protocols allow for variable-sized messages to accommodate different types of information.