Final answer:
The Sentinel value is used as a signal to mark the end of a list of values in programming, typically when the list's size is unknown. Therefore, the correct answer is option a. sentinel.
Step-by-step explanation:
The "Sentinel" represents a special value that marks the end of a list of values. When programming, it is common to use a sentinel value to signal the termination of a loop, indicating that there are no more data points to process. This is often used when the number of items in a list is not known in advance, and a special value, which would otherwise not be part of the dataset, is used to indicate the end of input.