In order for the function to represent an exponential decay, the value of b needs to be a value between 0 and 1.
So analysing each value, we have:
√(0.9)
Since 0.9 is lesser than 1, its square root is also lesser than 1, so this is a valid option.
1 1/5
This value is greater than 1, so it's not a valid option.
√e
The value of e is approximately 2.71, so its square root is greater than 1, so it's not a valid option.
2^-1
This value is equal to 1/2, that is, 0.5, so it's lesser than 1, therefore it's a valid option.
2-0.9999
This exp