a. I have to find the transfer characteristic
b. Remove D2 and R is moved to replace D2 find the transfer characteristic

a. The first one
when vi < 10V then D1 is reverse biased and D2 forward biased the circuit then becomes

Vout = 10V

Vi > 10V , Vout = Vin



b.

when Vi < 10V the D2 is open circuit Vout is 10V
when Vi > 10V the Vi is applied at Vout
I don't find any difference between a and b. Is it correct or missing something?

 

As per this schematic, there is no difference if we presume ideal voltage sources and ideal diodes.