Answer
Exponential decay.
Work Step by Step
The given function is
$y=(0.9)^{t-4}$
Use quotient of powers property.
$y=\frac{(0.9)^{t}}{(0.9)^4}$
Evaluate the power and simplify.
$y=\frac{(0.9)^{t}}{(0.9)^4}$
$y=1.52(0.9)^{t}$
$y=1.52(1-0.1)^{t}$
Hence, the function represents exponential decay.