Claudius Describe the new King's demeanor, his behavior toward other characters, and their reactions to him. How does
he provide assurance that the country is secure, and
he is the best-suited successor to his brother, King
Hamlet?

Iwas late to a meeting yesterday, and by the time i arrived, everyone  had left. 
you cannot choose  leaves  because the whole sentence is in the past tense, so present is not an option.
left is also a bad option because of sequence of tenses - what happened first is that they had left, and then you arrived.