The correct option is B As
A conjunction's main function is to join or combine two clauses as a statement. In the given sentence, the comma makes it easier to identify the clauses "He was ill" and "I postponed the meeting. The former is a subordinate/dependent clause, as it depends on the latter which is the main/independent clause, for its full meaning. 'As' combines these clauses, without which they don't look like they're one sentence. 'As' refers to the subordinate clause as a cause or reason for the action in the main clause "postponed" takes place. Hence, it's a conjunction and option D is correct. 'Ill' is an adjective as it modifies the pronoun "he" by describing him as 'someone who's suffering from a disease or illness'. It does not combine the clauses, hence option A is incorrect. 'He' is a pronoun of the third person singular form, pronouns modify nouns which have been identified or mentioned before. It does not connect the clauses, as the group of words before and after "he" are both incomplete, hence it's not a conjunction and option B is incorrect. "The" is an article that modifies the noun that follows. Articles signal that a noun is to follow, and that's the function of 'the'. It does not connect the clauses, as it's an article. Hence, option C is incorrect.