Hi,
When we talk about error, we usually dont refer to some level as being part of the error itself. For example, if this were a feedback loop that regulated voltage at say 10v output but actually put out 10.010v we would not refer to the error as 10v or 10.010v, but it would be 0.010v.
That means i would have said zero (0) because that would be the error whether or not it was 90 degrees or not, because if we expect 90 degrees and we get 90 degrees then that means there is zero error.
Interestingly, it's not, strictly speaking, zero either really, but depends on the gain in the circuit, but it could be very very close to zero even with some reasonable gain not considered to be exceptionally high.
I could be wrong there though, as they could in fact consider 90 degrees to be the error as Carl suggests. These kinds of questions sometimes depend on what the 'instructor' is looking for as part of the course material.