The blank requires a conjunction to denote that for the reason the coffee maker is kept aside. 'Because' (for the reason that; since), 'yet' (but at the same time), 'but' (used to introduce a phrase or clause contrasting with what has already been mentioned) and 'or' (used to link alternatives) are contextually irrelevant words and they be used in the blank. So, they are negated. Option (a) 'so' (and for this reason; therefore) is the most appropriate conjunction that fits the context of the sentence and aptly fits the blank too.
Hence, the correct answer is option (a).
Note:
CONJUNCTIONS
A conjunction (also called a connective) is a word such as and, because, but, for, if, or, when etc. Conjunctions are used to connect phrases, clauses, and sentences.
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