The correct option is B obscure
The word 'obvious' refers to something predictable and lacking in subtlety.
Option A, false refers to something not according with truth or fact.
Option B, obscure refers to something that is not clearly expressed or easily understood.
Option C, uncertain refers to something not known or definite.
Option D, difficult refers to something needing much effort and skill to accomplish.
Thus, according to the meaning, the word 'obscure' is the opposite of the word 'obvious'.