Answer
${Q}/{\epsilon_0}$
Work Step by Step
According to Guassian law of closed surfaces, the electric flux is given by
$$\Phi_e=\dfrac{Q_{in}}{\epsilon_0}$$
The given conductor is neutral but there is a charge of $Q$ inside its cavity, so the total inside charge of the closed surface is $Q_{in}=Q$.
Hence,
$$\boxed{\Phi_e=\dfrac{Q}{\epsilon_0}}$$
