The "turn on" voltage, also known as the forward voltage drop or knee voltage, is the voltage across a diode at which it begins to conduct significant current in the forward-biased direction. This voltage depends on the semiconductor material used to construct the diode. For a silicon diode*, the typical turn-on voltage is approximately $0.7 \text{ V}$. For a germanium diode*, the typical turn-on voltage is approximately $0.3 \text{ V}$. The question asks for the approximate turn-on voltage for a germanium diode. Comparing this with the given options: A) $0.7 \text{ V}$ (Silicon diode) B) $1.2 \text{ V}$ C) $0.3 \text{ V}$ (Germanium diode) D) $2.4 \text{ V}$ The correct option is C. The final answer is $\boxed{\text{C}}$.