" For a long time yesterday, he ......... in his office."
a- worked b- was working
Hello, Ahmed and Ahmed—I agree with Ahmed_btm that both answers are grammatically correct. The sentences essentially express the same thing, but from a slightly different viewpoint. Each has a slightly different "flavor."
It would be natural to use "For a long time yesterday, he worked in his office" if the speaker wanted to emphasize the location of his work during that time period.
- Lately, he has been out meeting with clients a lot. But for a long time yesterday, he worked in his office.
It would be natural to use "For a long time yesterday, he was working in his office" if the speaker were surprised or "taken" by his activity during that time period.
- He hasn't been feeling well lately. Sometimes he stays in bed all day. But for a long time yesterday, he was working in his office.