The given sentence talks about the writing of a particular person. Since no word in the sentence indicates which time period we are talking about, we will have to go through the options and assess which one is right.
Option A: It is incorrect. Since the subject within the sentence is singular, the verb will have to contain the suffix "s". The given option is inconsistent with the given rule and hence it is incorrect.
Option B: It is incorrect. Since the subject within the sentence is singular, the auxiliary verb can only be "is" and not "are". The given option is inconsistent with the rule and thus, it is incorrect.
Option C: It is correct. "Wrote" represents the past form of "Write" and past forms attached with the singular subject do not need any auxiliary verbs. Thus, "William wrote" is a meaningful sentence and indicates that "William used to write many great poems on nature". It is consistent with the rules of grammar and hence is correct.
Option D: It is incorrect. Since the subject within the sentence is singular the auxiliary verb can only be "has" and not "have". The given option is inconsistent with the rule and hence is incorrect.