In a photoelectric experiment, the minimum negative potential applied to the collector plate at which the photocurrent just drops to zero is called the

1. syllabus defines the relevant potentials. The collector plate $A$ is gradually made more negative with respect to the emitter $C$. 2. As the retarding voltage grows, slower photoelectrons cannot reach $A$, so the photocurrent falls. 3. The minimum retarding potential $V_0$ at which even the MOST energetic photoelectrons are just turned back, and the current becomes zero, is the STOPPING potential. 4. Option A (saturation) is the OPPOSITE limit, the positive potential at which all emitted electrons are collected. Threshold (B) refers to a frequency, not a potential. Ionisation potential (D) is an atomic-physics quantity unrelated to surface photoemission. _Source: syllabus Physics Part 2, Ch 11 "Dual Nature of Radiation and Matter", §11.4.2, p. 4 ¶3._