Select the option that will improve the highlighted part of the sentence. In case no
improvement is needed, select ‘No improvement’.
Herbalism had become an all but extinct skill in the Western world.

Correct option is C

The given sentence is grammatically and contextually correct and conveys the intended meaning therefore no improvement is required.
 Option a and Option b both change the meaning of the sentence and are grammatically incorrect.
 Option d is grammatically incorrect because it omits the auxiliary verb "had" which is necessary to form the past perfect tense and indicate that the skill of herbalism had become extinct in the past.