Step 1: Let the intercepts be \(A(a,0)\) and \(B(0,b)\).
Since \(P(3,5)\) divides \(AB\) internally in the ratio \(2:1\),
\[
P=\left(\frac{2\cdot0+1\cdot a}{3},
\frac{2\cdot b+1\cdot0}{3}\right)
=\left(\frac{a}{3},\frac{2b}{3}\right).
\]
Step 2: Find the intercepts.
Comparing coordinates,
\[
\frac{a}{3}=3
\quad\Rightarrow\quad
a=9,
\]
\[
\frac{2b}{3}=5
\quad\Rightarrow\quad
b=\frac{15}{2}.
\]
Step 3: Use the intercept form.
\[
\frac{x}{9}+\frac{y}{15/2}=1
\]
Multiplying throughout by \(45\),
\[
5x+6y=45.
\]
Step 4: Final conclusion.
\[
\boxed{5x+6y=45}
\]
Hence, the correct option is \(\boxed{(A)}\).