Answer
$\rm 216\;nm$
Work Step by Step
A strong reflection means that the reflected waves interfere constructively.
We have two main waves, the one that is reflected from the $\rm MgF_2$ film and the other one that is reflected from the glass, see the figure below.
And since the index of refraction of air is less than that of the film and that the index of refraction of the film is less than that of the glass$$n_{air}\lt n_{film}\lt n_{glass}$$
the two reflected waves indicate a $180^\circ$ phase change which means that they will be met in phase. And we need them to interfere constructively for orange light.
Hence, the thickness is then given by
$$2t= \dfrac{m\lambda}{n_{film}}$$
For the smallest thickness, $m=1$
$$t= \dfrac{\lambda}{2n_{film}}$$
Plugging the known;
$$t= \dfrac{600}{2(1.39)}$$
$$t=\color{red}{\bf 216}\;\rm nm$$
