Answer:
A. The number of pages in the assignment
Explanation:
You want to know which of several possibilities the amount of time it takes to complete a reading assignment might be a function of: pages, cost, age of the book.
Correlation
We expect that an assignment to read more pages will result in a longer completion time. While there might be different completion times for the same number of pages in different assignments (relation not a function), the relation might be a function for assignments in the same book.
There are many factors affecting the cost of a book, including the materials it is made of, the number of printed copies, the difficulty of providing the necessary typesetting and illustrations. These factors may also affect the time it takes to read an assignment. In general, we don't expect that a given number of pages in a book costing $20 will take longer to read than in a book costing $10 or $50. That is, we don't expect completion time to be a function of the cost of the book.
As with book cost, the year of publication may also have a relationship with readability. Older books may use different language, unfamiliar terminology, more (or less) complex illustrations, different typography. These may have a bearing on the time it takes to complete a reading assignment. However, if the same material is presented in the same way in a book published in 1900 versus 2000, we don't expect the assignment completion time to change.
The time it takes to complete a reading assignment is most likely to be a function of the number of pages in the assignment (other factors being equal).