Answer: The verbs that correctly complete the sentence are "cause" and "appears".
Step-by-step explanation: It is highly important that the verb of a sentence agree with its subject in number in order for the sentence to be correctly written. The sentence provided is made up of two independent clauses ("Comets or meteors cause craters" and "neither the planet's features nor its terrain appears rocky"). As the subject of the first clause is plural ("Comets or meteors"), the verb must also be plural; therefore, the correct option is "cause". Furthermore, as the second clause has been formed with a "neither...nor" structure and the subject nearest the verb is singular ("its terrain"), the verb must also be singular; therefore, the correct option is "appears".