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

The other options are not as accurate as tangible. Convincing proof is proof that is likely to make someone believe something. Strong proof is proof that is very convincing. Emphatic proof is proof that is stated in a very forceful way. Clear proof is proof that is easy to understand.

In the given example, the police need proof that is more than just convincing, strong, emphatic, or clear. They need proof that is tangible, meaning that it can be physically experienced. This is because tangible proof is the most reliable type of proof. It is less likely to be misinterpreted or fabricated than other types of proof.