If $f^{\prime}(x)=c f(x), c \neq 0 \& f(x) \neq 0$, then $f(x)$ must bean exponential function.

Step 1 ：The problem is asking if the derivative of a function is proportional to the function itself, then the function must be an exponential function. This is a well-known property of exponential functions. The derivative of an exponential function is proportional to the function itself. This is because the rate of change of an exponential function is directly proportional to the value of the function.

Step 2 ：To prove this, we can use the definition of the derivative and the property of exponential functions.

Step 3 ：Final Answer: Yes, if \(f^{\prime}(x)=c f(x), c \neq 0 \& f(x) \neq 0\), then \(f(x)\) must be an exponential function. This is because the rate of change of an exponential function is directly proportional to the value of the function. This property is unique to exponential functions.

Step 4 ：\(\boxed{Yes, if \(f^{\prime}(x)=c f(x), c \neq 0 \& f(x) \neq 0\), then \(f(x)\) must be an exponential function.}\)