Answer
$\text{exponential decay;}$
$y$-intercept = $100$
Work Step by Step
Recall:
The exponential function $y=c \cdot b^x$ involves exponential:
(i) growth when $b\gt 1$
(ii) decay when $0 \lt b \lt 1$
The given exponential function has $b=0.25$, which is between $0$ and $1$, therefore it involves exponential decay.
The $y$-intercept of a function can be found by setting $x=0$ then solving for $y$.
Set $x=0$ then solve for $y$ to obtain:
\begin{align*}
y&=100(0.25)^x\\
y&=100(0.25)^0\\
y&=100(1)\\
y&=100
\end{align*}
Thus, the $y$-intercept is $100$.