Step 1: Concept
The normal to the plane is the vector joining the two points. The plane passes through the midpoint.
Step 2: Meaning
Midpoint $M = (\frac{1+3}{2}, \frac{2+4}{2}, \frac{3+5}{2}) = (2, 3, 4)$. Direction ratios of the normal $\vec{n} = (3-1, 4-2, 5-3) = (2, 2, 2)$, which is equivalent to $(1, 1, 1)$.
Step 3: Analysis
Equation of plane: $1(x - 2) + 1(y - 3) + 1(z - 4) = 0$.
Step 4: Conclusion
$x - 2 + y - 3 + z - 4 = 0 \Rightarrow x + y + z - 9 = 0 \Rightarrow x + y + z = 9$.
Final Answer: (A)