Within a temperature range of +50C and -20C the VSI is Accurate to within limits of :
- A)+- 200 ft/min
- B)+- 0 ft/min
- c)+- 75 ft/min
- D)+- 300 ft/min
So you know the correct answer, but you want to know why... Well, the simple explanation is that is the accuracy limitations of that pressure based gauge. Every system has tolerances, that is the tolerance of that system.
In one of the airplanes I flew we had an digital instantaneous VSI that could be read from a display, and it came from a GPS coupled ring laser gyro INS. It was too jumpy to be useful, but on a stable descent it was nice to be able to cross check it against the pressure VSI.
They were usually pretty close, but there was always some slight difference. Assuming that the instantaneous VSI was more accurate, (it would also have some tolerance...) then that difference would be the accuracy limit of the gauge.