Original sentences: sir edmund was anxious to explore antarctica. so was his son, cecil.
combined sentence: sir edmund and his son, cecil, were anxious to explore antarctica.
does the combined sentence use a compound subject, compound verb, or compound object?
compound verb
compound object
compound subject

The correct answer is C. compound subject.

If you compare the two sentences above, you will see that the combined one combined the two subjects - Sir Edmund and his son, Cecil. Both words are the subjects of their respective sentences, but when you combine them into one subject using the conjunction and, it turns from a simple subject into a compound subject. To sum up, a compound subject contains more than one item connected by the word and.

Given that nothing changed in the verb was/were (it is still simple) and the object Antarctica (it is also still simple), those two cannot be correct answers.