The correct answer is the word 'just'.
Adverbs are words or phrases that modify or change the verb, adjective, determiner, noun phrase, sentence or clause they refer to. Adverbs usually express frequency, degree, level of certainty, time, manner or place and answer questions like "in what way?", "to what extent" etc. It is noted by modern linguists that adverbs tend not to fall under any other word category. In this case 'The' and 'a' are determiners; 'king' and 'ruler' are nouns; 'is' is a verb and 'just' is an adverb.
Lady Macbeth<span> starts off innocent until reading the letter from </span>Macbeth<span>, telling her about the witches' prophecies. She quickly changes and becomes cold-blooded and starts plotting against Duncan. She persuades </span>Macbeth<span> and pushes him into doing the deed</span>
