Let the efficiency of $S, E$ and $F$ be $r$ units/hour.
So the efficiency of $M = 2r$ units/hour.
Let the number of days $E$ works be $d$, so that of $M$ is $d/2$.
The amount of work done by $M,$ $W_{m} = 2r \times (d/2) \times 6.$
(As $M$ is working $6$ hours per day)
The amount of work done by $E,$ $W_{e} = r \times d \times 12.$
$W_{m}$ : $W_{e}$ = $\frac{2r \times (d/2) \times 6}{r \times d \times 12} = 1 : 2.$
Correct Answer: $B$