Step 1: Find the value of \(a\).
The given equation is
\[
5x+3ay+k=0.
\]
Its slope is
\[
m=-\frac{5}{3a}.
\]
Given,
\[
-\frac{5}{3a}=\frac{10}{9}.
\]
Hence,
\[
45=-30a
\]
\[
a=-\frac32.
\]
Step 2: Substitute the point \((4,8)\).
Substituting \(a=-\dfrac32\),
\[
5x-\frac92y+k=0.
\]
Using the point \((4,8)\),
\[
5(4)-\frac92(8)+k=0
\]
\[
20-36+k=0
\]
\[
k=16.
\]
Step 3: Final conclusion.
\[
\boxed{k=16}
\]
Hence, the correct option is \(\boxed{(C)}\).