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.
