The correct option is D the greatest
The sentence is in the superlative degree when the noun 'king' has the quality of the highest degree, the adjective of quality being 'great'. The superlative form of 'great' is 'the greatest', hence option D is correct. The phrase 'as great as' is in the positive degree, since the sentence is supposed to be of the superlative degree, option A is wrong. Option B is incorrect because the phrase 'the greater' has the comparative 'greater' and 'the' is the article that modifies a superlative, which don't go together. Option C 'great' does not refer to any degree of comparison by itself.