Ans: B
Let $2^pa^qb^rc^s$ be the third expression.
Let $2^pa^qb^rc^s$ be the third expression.
$\begin{array}{rlllllll}
\mbox{1st expression} = & 2 & \times & a^2 & \times & b^4 & \times & c \\
\mbox{2nd expression} = & 2^2 & \times & a^4 & \times & b^2 & \times & c^6 \\
\mbox{3rd expression} = & 2^p & \times & a^q & \times & b^r & \times & c^s \\ \hline
\mbox{H.C.F.} = & 2^0 & \times & a & \times & b^2 & \times & c^0 \\
\mbox{L.C.M.} = & 2^2 & \times & a^4 & \times & b^5 & \times & c^6
\end{array}$
Therefore, $p=0$, $q=1$, $r=5$ and $s=0$. i.e. the third expression is $ab^5$.
