$\dfrac{DMTE}{W}=C$(Constant)
Where, $D=$ Days, $M =$ Man Power/machines, $T =$ Time , $E =$ Efficiency, $W =$ Work
So, $\dfrac{M_{1}T_{1}}{W_{1}}=\dfrac{M_{2}T_{2}}{W_{2}}$
$\implies \dfrac{7\times 7}{7}=\dfrac{100\times T_{2}}{100}$
$\implies T_{2}=7$ minutes
So, the correct answer is $(B).$