For this case we have functions of the form: 

Where 
- A: initial amount 
- b: rate of change 
- x: independent variable 
- y: dependent variable 
If 
 : the exponential function decreases
: the exponential function decreases 
if 
 : the exponential function grows
: the exponential function grows 
We have then that the following exponential function decreases:
 

Answer: 
an example of exponential decay is: 

Option C