Answer
110 nm.
Work Step by Step
There is no net phase difference introduced because both rays reflect from identical substrates. The condition for destructive interference is $2t=(m+\frac{1}{2})\lambda$.
For the minimum pit depth t, m=0.
$$2t=(0+\frac{1}{2})\lambda$$
$$t=\frac{\lambda}{4} $$
Note that $\lambda=\frac{\lambda_0}{1.8}$.
$$t=\frac{\lambda_0}{4(1.8)}=\frac{790nm}{4(1.8)}=110 nm $$
