For these blanks, we need words describing the qualities in the second part of the sentence: one of them should have a meaning similar to "resolute," which means "firm, unhesitating." The second one should signify politeness.
Among the answers, we have two first words that signify firmness: choice (B), "steadfast," and choice (D), "tenacious." "Steadfast" means "firm in belief and determination," while "tenacious" means "not easily stopped, very determined," so either of the words could be used here. However, the second word in choice (B)—"manipulative"—doesn't fit in this context, since we're looking for the word associated with politeness. The second word in choice (D)—"courteous"—means "very polite in a way that shows respect," and it fits perfectly.
The correct answer is (D).