The park ranger__the tourists around the camp site to investigate the area.

B. Guides

Explanation:

The park ranger is a singular subject, so instead of saying "park ranger" replace it with he or she, then add the words to see if they sound correct.

He/she guide the tourists doesn't sound quite right. It would sound better if the subject was plural. Ex: The rangers guide the tourists...So this eliminates answer A.

He/she guides the tourists sounds grammatically correct because it is.

Nah i'm just kidding. It sounds correct because the subject is singular. When you have a singular subject, the present tense verb must be plural. Ex: He walksIt climbsLilah runs. The rule applies to all of them.

P.S. Don't forget that the verb always applies to the subject in front of it, not behind it. So don't pit the words "guide/guides" with "the tourists" because you might get it wrong.