Answer
$189nm$
Work Step by Step
We can find the required minimum thickness of the oil film as follows:
$t_{min}=\frac{m(\lambda)}{2n}$
We plug in the known values to obtain:
$t_{min}=\frac{(1(521\times 10^{-9}))}{2(1.38)}$
$t_{min}=188.76\times 10^{-9}m=189nm$