Final answer:
The collection of natural numbers greater than one quadrillion is not a well-defined set and therefore not a set.
Step-by-step explanation:
The collection of natural numbers greater than one quadrillion is not a well-defined set and therefore not a set. A set is considered well-defined if its elements can be clearly and unambiguously determined. However, in this case, there is no clear and unambiguous way to determine the collection of natural numbers greater than one quadrillion, as it is an infinite set and has no defined endpoint. Therefore, the answer is option b) It is not a well-defined set.