The correct option is D Clear
The words 'obscure' refers to something not clearly expressed or easily understood.
Option A, clear refers to something easy to perceive, understand or interpret.
Option B, dogged refers to having or showing tenacity and grim persistence.
Option C, decent refers to a respectable and accepted standards of moral behaviour.
Option D, sensible refers to being in accordance with wisdom or prudence.
Thus, according to the meaning, the word 'clear' is the farthest in meaning from the word 'obscure'.