The correct option is B is always the best policy
Tense is used to provide more information about an action ( its time, completeness, etc.). Option C)is always the best policy is correct as the principal cause uses the simple past tense and therefore, the subordinate clause can use simple present, simple past, or past perfect tense and as the sentence talks about a universal truth, present tense must be used. The other options are wrong as no other tense is appropriate for the given sentence. The correct answer is C)is always the best policy.