Negative feedback circuit problem!
I think, Sven B has answered your questions as far as (a) and (c) are concerned.
Here is my answer for (b):

To identify the open-loop gain A, you must open the loop at the gate node - hence, the value of A is determined by RD||(R1+R2).   

The feedback factor is simply β=R2/(R1+R2) and the loop gain is LG=βA.

Comment (feedback principle)

I must admit that I do not like at all terms like "series-shunt". Sometimes this may sound confusing because some authors use the input-output sequence and some authors prefer the output-input sequence. 

In the present case we have series-shunt feedback (input-output), which means: Voltage-controlled voltage feedback. That means: The feedback signal is derived from the output voltage of the active device and is fed back in form of a voltage.  

Counter example: For an inverting opamp we have voltage-controlled current feedback because we have two currents which meet in a common node (input current and feedback current are superimposed at the inverting opamp input).