Calculation of output impedance of CE emitter bias configuration( unbypassed) with r_0
Srijita, what you need to know here that this amplifier has feedback because of resistor RE.  The feedback is in series with the output so increases the output resistance of the transistor, not including RC.    If you removed this feedback by putting a big capacitor across RE, the answer would simply be ro||RC.  The way you solve this is to put a 1A current source across RC.  Then do node analysis to get the voltage across RC.  That voltage divided by one is the output resistance.  You see that part of the 1A source will go through ro and RE raising the voltage at e.   When this happens the controlled source will decrease reducing how much current the transistor takes, which makes ro look bigger.