In the conjunctive adverb, an adverb connects the idea in a sentence. Whereas, conjunctions connect the words, clauses or phrases to clarify the meaning of the sentence.
Some of the examples of conjunctive adverbs are, accordingly, however, otherwise, instead and so forth.
A conjunctive adverb helps in connecting two main clauses and behaves like a coordinating conjunction. To connect clauses it uses a semicolon to connect. Hence, the formation is as follows,
Main clause +; + conjunctive adverb + main clause.
Therefore, in the above-given sentence, a conjunctive adjective which best completes the sentence is meanwhile.
Marcos chopped onions for the soup; meanwhile, Maria prepared the cheese plate.
Herein, meanwhile helps in connecting two actions that went simultaneously.