The correct option is B natural
The word 'artificial' refers to being made or produced by human beings.
Option A, genuine refers to being truly what something is said to be.
Option B, natural refers to being existed or derived form nature.
Option C, true refers to something in accordance with fact or reality.
Option D, authentic refers to being accurate and reliable.
Thus, according to the meaning, the word 'natural' is the opposite of the word 'artificial'.