Mike was concerned ________ Katalina's suitcase was left by the door.

A sentence diagram. A horizontal line has the words Mike was concerned. A dotted line with a question mark label leads from the words was concerned to a horizontal line with the words suitcase was left. Below suitcase is the word Katalina's. Below the words was left are the words by door. Below by door is the word the.

Based on this sentence and the sentence diagram, which conjunction best completes the sentence?

which
so that
because
whereas