The correct option is
C Superficial
We would be choosing the word opposite to the meaning of "Radical" from the options given below
Radical means basic, new and different from the obvious. Thus, the opposite would be something not complete or something that is just the obvious
Now, if we look into the given options,
Uncompromising means not willing to change a decision
Basic relates to the easiest part of something. It is nearly synonymous to radical
Thorough means marked in full details. It is nearly synonymous to radical
Superficial means over the surface or sometimes which states the obvious. It is the correct option
Option D is correct