Answer:
The subject of the sentence "His father was a king over a great land" is "his father."
Step-by-step explanation:
The subject is the topic of a sentence, what the sentence is about, what it talks of. The subject is supposed to have a "relationship" with the verb, that is, the verb has to agree with the subject - singular or plural, for instance.
The sentence "His father was a king over a great land" is talking about "his father". The verb "was" agrees with the subject - third-person singular. The rest of the sentence after the verb is what we call predicate. It consists of information about the subject. Thus, we are talking about "his father" - which makes it the subject -, and we are saying that he "was a king over a great land" - verb +predicate.