The correct option is A that
A relative pronoun refers or relates to a noun that's mentioned before, it also functions as a conjunction, it combines together two sentences. Here, the sentences 'He was the king of all the lands' and 'He could view all the lands' are combined together as the sentence given. In the given sentence, "he" is the subject as he performs the action "view" towards the object "the lands". Here 'that' refers to the object "lands", which is the antecedent. It also seems to combine the two sentences. Hence, 'that' is the relative pronoun and option A is correct. 'Lands' is the antecedent (a noun) which the relative pronoun refers to. It isn't the relative pronoun, hence option B is incorrect. 'He' is the subject as it performs the action "view" and does not refer to the antecedent, hence option C is incorrect. 'King' is a noun that the relative pronoun 'that' is not referring to. Hence option D is incorrect.