Exponential decay is always represented by a curve that's
going down on the far right, approaching a horizontal line,
usually the x-axis. 

1.



No, that's going up on the right, so it's not going down 
approaching a horizontal line on the far right, so it's not 
decaying.

2.



No, that's going up on the right very slowly, so it's not 
going down approaching a horizontal line, so it's not 
decaying.

3.



No, that's going up on the right, so it's not going down 
approaching a horizontal line, so it's not decaying.

4.



Yup, that's going down approaching the x-axis,
so it's the only one decaying.

The rule is this. If its equation is of the form:

, where a is positive and not 1, and
b is not 0.

then 

if a > 1, then b < 0 (b must be negative) in order to have 
exponential decay (approaching the x-axis on the right from 
above).

if a < 1, then b > 0 (b must be positive) in order to have 
exponential decay (approaching the x-axis on the right from 
above).

Edwin