Ans: A
$\begin{array}{rcl}
(\alpha +\beta): (3\alpha-\beta) & = & 7:3 \\
\dfrac{\alpha+\beta}{3\alpha-\beta} & = & \dfrac{7}{3} \\
3\alpha +3\beta & = & 21\alpha -7 \beta \\
-18\alpha & = & -10 \beta \\
\dfrac{\alpha}{\beta} & = & \dfrac{5}{9} \\
\alpha : \beta & = & 5 : 9
\end{array}$
$\begin{array}{rcl}
(\alpha +\beta): (3\alpha-\beta) & = & 7:3 \\
\dfrac{\alpha+\beta}{3\alpha-\beta} & = & \dfrac{7}{3} \\
3\alpha +3\beta & = & 21\alpha -7 \beta \\
-18\alpha & = & -10 \beta \\
\dfrac{\alpha}{\beta} & = & \dfrac{5}{9} \\
\alpha : \beta & = & 5 : 9
\end{array}$
