Final answer:
In the context of sequences, which are functions with a domain typically consisting of natural numbers, the only possible value from the given options that could be part of the domain is 7, as it is a non-negative integer.
Step-by-step explanation:
When recognizing that a sequence is a function, it's important to understand that the domain of a sequence is typically the set of natural numbers {1, 2, 3, ...} or sometimes starting from zero {0, 1, 2, ...}, because a sequence is a function that associates a unique element of the range with each natural number. Therefore, the only elements that could be a part of the domain of such a function are non-negative integers. Among the given options, a. 7 and b. -3, the correct answer would be option a. 7, because -3 is a negative number and does not belong in the set of natural numbers, whereas 7 is a non-negative integer and fits the typical domain criteria for sequences.