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.