Explanation: The "effectiveness of the grid in causing changes of plate current" describes how much the plate current changes for a given change in grid voltage. This critical characteristic is precisely defined by **transconductance (gm)**. Transconductance is the ratio of the change in plate current (ΔIp) to the change in grid voltage (ΔVg) that caused it, with the plate voltage held constant (gm = ΔIp/ΔVg). It measures the grid's ability to control the electron flow. **Mutual conductance** is an older, but equivalent, term specifically used in the context of vacuum tubes to describe the exact same parameter. Both terms quantify the grid's controlling influence over the plate current. **Plate resistance (rp)**, however, is the internal AC resistance of the tube between the plate and cathode (rp = ΔVp/ΔIp at constant Vg), not the grid's control effectiveness. Therefore, both transconductance and mutual conductance accurately describe the concept.