Fill in the blank with the appropriate verb tense:
Copernicus rightly observed that the planets _________ around the Sun.
The correct option is C revolve
Tense is used to provide more information about an action ( its time, completeness, etc.). Option C)revolve 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 general fact, 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)revolve.