Ans: B
$\begin{array}{rcl}
a(x+3)+b(3x+1) & \equiv & c(x+2) \\
ax +3a +3bx +b & \equiv & cx +2c \\
(a+3b)x +(3a+b) & \equiv & cx + 2c
\end{array}$
By comparing the coefficients of both sides, we have
$\left\{ \begin{array}{l}
a+3b = c & \ldots \unicode{x2460} \\
3a+b = 2c & \ldots \unicode{x2461}
\end{array}\right.$
Sub. $\unicode{x2460}$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
3a+b & = & 2(a+3b) \\
3a+b & = & 2a+6b \\
a & = & 5b \\
\dfrac{a}{b} & = & 5 \\
a:b & = & 5:1
\end{array}$
