The police need tangible proof of his guilt before they can act against him.

Answer: Option D

Convincing : capable of causing someone to believe that something is true or real.
Strong : having the power to move heavy weights or perform other physically demanding tasks.
Emphatic : expressing something forcibly and clearly.
Clear : easy to perceive, understand, or interpret.
Tangible : clear and definite; real.

Synonym of Tangible is Clear