[أحمد العبدالله]

Thank you my sister Al- Asilah

In fact I have two comments on your answers:

A- As to #5 : He is sitting at his :

a: chair b: desk c: office d: room

and you said the ideal answer is c: office.

I have three points:

1- If you read the previous sentences and imagine the man holding a piece of paper surely you will imagine him sitting at his desk because the word desk is more suitable for this situation.
2- If he is really at his office , there is no need to use the word perhaps in number 6 because if he is really at his office, he is surely at work or on the job.(supposing that we can use at with the word office)
3- I don't think that we can use the preposition (at) with the word office because the preposition( in) is commonly used with the word office while I am so sure of the use of (at )with the word desk.( Fot more information check the book of Michael Swan , Practical English Usage)

B: As to #6 in fact I wanted to choose D: on job but when I looked it up in the dictionary, I found that it is not complete because it should be on the job with the added ,that is why I chose at work.

I hope that you read my reply and reply to me.

I wish you all the best.

Ahmad