Correct Answer: unless

Solution : The fourth option is correct.

• Let us explore the given options:
  • The adverb "still " means even now (or then) as formerly .
  • The adverb "whenever " means at whatever time or on whatever occasion .
  • The conjunction "unless " means except if (used to introduce the case in which a statement being made is not true or valid) .
  • The adverb "otherwise " means in circumstances different from those present, considered, or else .

Hence, the complete sentence is: "The police will regard it as an accident unless any evidence to the contrary is found."