A sentinel value and signals that there are no more values to be entered:
a. is a different data type than the values being processed
b. is a special value that cannot be mistaken as a member of the list
c. indicates the start of a list
d. guards the list

The answer is "Option b".

Explanation:

Sentinel [..] is a special value, which existence ensures the end of a loop, which performs structured data. It allows the information end to be identified where no other means are available for this, and wrong choices can be described as follows:

In option a, It is a special value, not a data type, that's why it is not correct. Option c and Option d both were wrong because, It is an empty list, and it can't indicate the starting point of the list and guards the list.  

Answer (a) primarily in your lane before entering the shoulder;

The answer for your specific question is d.