The office opens at 10 is continuous. (The office opens at 10 every day) The office is open at 10 could signify a once only event.(The office is open at 10 today only) The office is opened/is being opened signifies a short duration: (the office is opened at 10 for one hour only, then shut)
No, "is open" is different from "is opened at".
"The office is open at 10 o'clock" is a statement.
"The office is opened at 10 o' clock" is in the passive voice; it places emphasis on the action of the store being opened at 10 o'clock.
The skill being tested here is the "passive voice" after all.