Explanation: The question describes the definition of a vacuum tube's amplification factor, also known as mu ($\mu$). The amplification factor is a fundamental parameter representing the theoretical maximum voltage gain a tube can provide. It's the ratio of a change in plate voltage (ΔVp) to a change in grid voltage (ΔVg) that produces the same change in plate current (ΔIp), with the current held constant. In simpler terms, it indicates how much more effectively the grid voltage controls the plate current compared to the plate voltage itself. Power factor (B) relates to AC circuit efficiency and the phase relationship between voltage and current, not a tube's inherent gain. Maximum inverse plate voltage (C) is a tube rating specifying the maximum reverse voltage the plate can withstand, primarily relevant for rectifier tubes, and has no bearing on gain.