Step 1: Use the derivative of the inverse function.
If
\[
g=f^{-1},
\]
then
\[
g'(y)=\frac{1}{f'(x)},
\]
where
\[
y=f(x).
\]
Differentiating once again,
\[
g''(y)
=
-\frac{f''(x)}{[f'(x)]^3}.
\]
Step 2: Use the given condition.
Since
\[
f(0)=a,
\]
we have
\[
g(a)=0.
\]
Hence, in the above formula, substitute
\[
x=0.
\]
Therefore,
\[
g''(a)
=
-\frac{f''(0)}{[f'(0)]^3}.
\]
Step 3: Write the final answer.
Hence,
\[
\boxed{
g''(a)
=
-\frac{f''(0)}{[f'(0)]^3}
}.
\]
Thus,
\[
\boxed{(A)}
\]
is the correct answer.