For the approximate turn-on voltage for a germanium diode.

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 $\boxed{\text{C}}$.