The correct answer is: C. Not fall below 0.7 V.
Germanium transistors have a base-emitter voltage drop of about 0.7 V. This means that in order for the transistor to amplify a signal, the voltage between the base and the collector must be greater than 0.7 V. If the voltage between the base and the collector falls below 0.7 V, the transistor will not amplify the signal and the output will be very small.
Option A is incorrect because the base-emitter voltage drop is not zero. Option B is incorrect because the base-emitter voltage drop is not 0.2 V. Option D is incorrect because the base-emitter voltage drop must be greater than 0.7 V for the transistor to amplify a signal.