A transistor may be used as a switching device or as a:

ANS:D - variable resistor

Case 1: If transistor is in OFF condition collector current is High. output voltage Vo = logic 1 = Zero or Min resistance path. Case 2: If transistor is in ON condition collector current Ic is LOW.because (Ic =Ib+Ie) total current goes to emitter terminal or ground. So output voltage Vo=logic 0 = Max resistance path. Both cases resistance is variable Min(0) to Max(1).