Connector Choice
Only one of these connectives joins the two halves truthfully. Drop one in and read the whole sentence back before you decide.
The finished sentence is "Wait in the shelter until the bus arrives." The two halves are linked by a point in time, and "until" is the connective that signals it. A connective is not decoration between two clauses: it is the word that tells the reader what the second half is doing to the first, so swapping it changes what the sentence claims rather than only how it sounds. Each of the others fails for its own reason. "so that" signals a purpose, and what the second half does to the first here is give a point in time; "unless" signals a condition, and what the second half does to the first here is give a point in time; "although" signals a contrast, and what the second half does to the first here is give a point in time.
What to say if they get stuck
- Ignore the chips for a moment. Ask what the second half is DOING to the first: giving a reason, setting a condition, marking a time, or turning it around.
- Every chip is a real joining word, so none of them looks odd on its own. The question is only whether it describes the link these two particular halves have.
- Read it back with each chip in place. "Wait in the shelter until the bus arrives" says a point in time, which is what this pair means.