The correct answer is meanwhile.
Transition words are used to connect ideas and paragraphs within texts. They are there to help unify the text, clarify the connections between ideas and sentences, and overall provide a smooth transition from one idea onto the next one.
When it comes to the options above, meanwhile is the only transition word that has anything to do with sequence. It shows us what happened while another thing was happening. The other options don't have the same idea.
