This is a boolean variable that signals when some condition exists in the program: A. Sentinel B. Block C. Flag D. Case

Answer: C) Flag

Explanation: Flag is the variable that is used at time of the condition when the it becomes true. It is the variable that checks the condition and also to handle the function flow any as as response gives a single value.

Other given option includes case that describes the case statement, sentinel variable comes into action at the time of function termination and block variable are the variables for creating the blocks in function. Thus the correct option is option (c).