Functions will not always be linear or exponential but in this question there is 1 linear and 3 exponential answer choices:
An exponential function has the variable on the exponent.  A student in a classroom last year called it the “floating midget x”!
Let’s look at our options:
(1) compounded interest: exponential!!  Notice the formula has A = P (1+r)^t — that exponent makes it exponential and not linear!!
(2) doubling…hey that’s Moore’s Law: exponential function that makes the phone/computer you are reading this on exist!!
(3) cell phone service we could write the equation like this:  y = .20x + base  looks mighty like y = mx + b
(4) decay function is exponential as it also uses an exponent