No option is correctIf we take $$\\frac{N_{A}}{N_{B}}=\\frac{1}{e}$$Then$$\\frac{N_{A}}{N_{B}}=\\frac{e^{-8 \\lambda t}}{e^{-\\lambda t}}$$ $$\\begin{aligned} \\frac{1}{e} &=e^{-7 \\lambda t} \\\\ -1 &=-7 \\lambda t \\\\ t &=\\frac{1}{7 \\lambda} \\end{aligned}$$