Chapter 4 - An Introduction to Functions - 4-2 Patterns and Linear Functions - Apply What You've Learned - Page 245: c

This relationship is definitely a function. A function is a relationship between inputs and outputs (variables such as months and subscribers) where an input has a specific output. In this case, the input is the number of months and the output is the number of subscribers. The input always gives a unique output, making it a function.

Explanation above.