Seen this question before.
Would definitely go for Oxford's answer - D
I think Bristol are trying to screw with your brain here. Think about, are you actually going to get attitude information from the unit that you've momentarily shut down? After all, it doesn't come with a pictorial view to display attitude - you look at the AI for that.
Both answers are correct, as the INS in the Oxford wording can still be used in Att Ref mode (when used in conjuntion with an AI).
Bristol's answer C is more correct than D, as they are specifically referring to IRS against using the unit for attitude information.
Oxford's wording is much more like what you'd come across in the writtens, so I'd focus more on that syle than Bristols'.
Difference between IRS / INS - you should know this if you're doing ATPL's!!
IRS - Utilises Ring Laser Gyro
INS - doesn't! Old version