a. why do the record deck, tuner, cd, and cassette deck all feed into the amplifier?
c. conecting cd to amplifier input or out put socket? why?
for part c. the why bit means why out put of cd in to the input and not the other way round?
The devices you mention are components that do not have amplifiers built into them (to provide sufficient power to the speakers).
A receiver/amplifier is a central hub that your devices can connect to. This is more efficient, otherwise you would need a set of speakers for every single device you wanted to listen to.
You would connect the output from your cd player to the input on the receiver. The receiver needs an audio signal to amplify and send to the speakers.
An amplifier takes a relatively weak electrical input signal (from the record deck, tuner, etc.) and makes it strong enough (amplifies) to run the speakers. All the components supply weak inputs and the amplifier supplies strong outputs to drive the speakers (or perhaps as inputs to other components such as a headset). Input means 'put in' here and output means 'puts out' here. Connect the output of the cd player to the input of the amplifier. This is often done internally by switches when the cd player and amplifier are in the same cabinet.