The correct answer is C. 3 mA.
The collector current due to signal alone is 3 mA. This means that when there is no signal applied to the transistor, the collector current is 0 mA. However, when a signal is applied, the collector current increases to 3 mA. Therefore, the zero signal collector current must be at least 3 mA in order to accommodate the maximum collector current due to signal alone.
Option A is incorrect because 6 mA is greater than the maximum collector current due to signal alone.
Option B is incorrect because 2 mA is less than the maximum collector current due to signal alone.
Option D is incorrect because 1 mA is also less than the maximum collector current due to signal alone.