The correct option is B unchangeable
'Immutable' means 'unable to be changed'.
Option C: 'Unchangeable' means the same thing: unable to be changed or altered. Hence C is the correct option.
Option A: 'Important' refers to something of great significance or value.
Option B: 'Unique' refers to something or someone who is rare, unlike the others.
Option D: 'Awful' refers to something as 'bad or unpleasant'.
These do not express the meaning of 'immutable'. Hence A, B and D are incorrect.