The correct answer is C. He wants answers to the mysteries of Stonehenge.
When a verb is correctly conjugated, that means that it is in the same person and number as the subject. So, if the subject is the pronoun he, then the verb will end in -s in the present simple tense.
A is incorrect because it should be is instead of be; B is incorrect because it should be visits instead of visit; D is incorrect because it should be plans instead of plan. The only option which is completely correct is C - the verb wants agrees with the subject he.