Final answer:
The sentence 'The surface, or photosphere, is frequently studied' is a simple sentence with a single subject and verb, rather than being a compound sentence with two independent clauses.
Step-by-step explanation:
Regarding the sentence in question, "The surface, or photosphere, is frequently studied" is a simple sentence. It consists of a single independent clause with a single subject and verb. In this case, the subject is "The surface, or photosphere," and the verb is "is studied." A compound sentence would require at least two independent clauses connected by a conjunction such as 'and', 'or', or 'but'. Here, the sentence does not fit that structure.