Solution:

The answer is (D). Consider a rectangular table in which the length $l$ is the minimum such that the three pizzas are entirely contained on its surface, that is, such that the second and third pizzas are tangent to the side at which none of the three friends is sitting. Let us call $O_{1}, O_{2}$ the centers of the first two pizzas, and let $P$ be the point of intersection between the line through $O_{1}$ parallel to the side to which the second pizza is tangent (which has length $l$) and the line through $O_{2}$ parallel to the side to which the first pizza is tangent (which has length $70~\mathrm{cm}$). Then $l=15+O_{1}P+15$; on the other hand $O_{1}O_{2}$ has length $60~\mathrm{cm}$, while the length of $O_{2}P$ can be computed, in centimeters, as $\frac{70}{2}-15=20$; the angle $O_{1}\widehat{P}O_{2}$ is right by definition, and thus by the Pythagorean Theorem $O_{1}P=\sqrt{O_{1}O_{2}^{2}-O_{2}P^{2}}$, that is, the required length turns out to be, in centimeters, $30+10\sqrt{5}$.