 Solution

A LIFO stack can make waiting clients starve. Suppose client $A$ waits at the bottom of the stack. Before each current owner releases, a fresh client submits a request. That newest request is pushed above $A$ and is selected next. Every animation can finish promptly while $A$ waits forever.

This is LIFO starvation: service completion alone does not ensure starvation freedom. A FIFO queue prevents newer same-priority requests from passing $A$, whereas replacing either priority-class FIFO queue by a LIFO stack adds this problem even within one priority class. Strict priority can already starve ordinary work, as in (b); the LIFO stack introduces a further unfairness that can also affect urgent work.