For a transistor amplifier, the voltage gain
(a) remains constant for all frequencies.
(b) is high at high and low frequencies and constant in the middle
frequency range.(c) is low at high and low frequencies and constant at mid
frequencies.(d) None of the above.
My attempt: probably naive, since there is more change in voltage at higher frequency it should be option (b) but this is wrong and the correct one is ____. ( I know the answer but not reason)
Any explanation at the level of high school(12th class) would be helpful.
Edit: after comments , the only amplifier circuit diagram discussed is given below 
Best Answer
Depends of the schematic, for just a transistor it lowers with frequency, but in tipical amplifier it is c). At lower frequencies if coupled with capacitor gain is low because of reactance of that capacitor. At high frequencies it is lower because of transistor gain. If it is DC coupled, than have low gain only on high frequencies.
http://elearning.vtu.ac.in/P9/notes/06ES32/Unit6-MSS.pdf
EDIT: after your update, definitely it is answer C. At lower frequencies you have losses in capacitor before Rb (at frequency 0, it have infinite reactance).
You can see input section: It consist from capacitor, resistor and base-emiter diode (you can think of it as a resistor in this case). Resistance (reactance) of capacitor is $$ Xc = \frac{1}{2\pi f C}$$ So resistance of capacitor changes (lowers) with frequency. At low frequencies it is very high, so input signal is divided between it, resistor and base junction and only small part of input signal is "seen" at base junction which is then amplified. Because of that you will have lower output signal at low frequencies.
At higher frequencies, input capacitor influence is negligible, so circuit amplification depends only on transistor which have lower gain at high frequencies.
In the middle reactance of the capacitor is small and also gain of transistor is high, so your gain would be high.
EDIT2: You also have capacitor on output which will also lower gain of low frequencies (similar as input capacitor because of high resistance (reactance)).