Most stores in large American cities close ______ five or six o’clock on weekdays, but the malls in the suburbs stay open much later.
Most stores in large American cities close ______ five or six o’clock on weekdays, but the malls in the suburbs stay open much later.
Explanation
"Close at five or six o'clock" is correct because "at" is used for specific times.
It precisely indicates when the stores close.