As Ian said, gramatically both are correct. But look at the meaning, If your car was going to be repaired this week, there was no reason for you to take the bus. If you are taking the bus, it simply means that you dont have your car with us, which in turn means that your car is out for repairs. So, My car is being repaired is the correct answer in this context. However, In a sentence like "My car (repair) ................. next Saturday, so I will have to go to office by bus", you will use "will be repaired".

yes, that is correct. My car is being repaired this week so I am going to work by bus. Look at it this way. A friend comes up to you and asked? Where is your car? And you would respond? It IS in the car shop being repaired. If its happening now, then use it is. You are taking the bus THIS week. Here and now. So where is your car, at this moment??? It is...in the car shop being repaired. Now you might want to add. It will be repaired by the end of the week. So one last example. My car is being repaired this week so I am going to work by bus. My car will be repaired by the end of the week.

Both are correct grammatically, but you have to look at the whole sentence's meaning. It's because of the second part - since the person is telling you that they have to work this bus this week, we know it must be the present time they are describing (it's being repaired now, so she has to take the bus, she doesnt have her car)