Final answer:
Among the options given, a) [Dogs with 6 eyes] represents a null set because there is no such thing as dogs with six eyes, making it a set with no elements.
Step-by-step explanation:
A null set, also known as an empty set, is a set that does not contain any elements. Among the options given, a) [Dogs with 6 eyes] represents a null set because, in reality, dogs do not have six eyes. Therefore, there are no elements (dogs with six eyes) that can belong to this set, making it a null set.
On the other hand, b) [Grains of sand on the seashore] is not a null set because there are indeed grains of sand on a seashore, c) [Kaydian, Betty, Pam, Jane] is a set containing the names of four individuals, and d) [N, u, l, l] is a set containing characters that spell 'null', but it is still a set with elements, and hence not a null set.