I have struggled for years to find an authoritative source for an operational test of altimeter accuracy in EASA operations. I can't find anything in EASA docs but if I have missed it I would be grateful for a reference. I have not looked at TSOs as they are pay-to-use, maybe there? If so it would be a calibration limit.
FAA TSOs appear to require a calibration accuracy of 20 ft with 29.92 set and within 1000 ft of sea level. FAR 91.411 appendix E appears to confirm that. The FAA Instrument Flying Handbook, however, suggests an operational limit (rather than a calibration limit) of 75 ft.
There is an underlying ICAO requirement for an operational test in PANS-OPS Part III Section 1 Chapter 3.2 which is:
A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this point, within a tolerance of:
a) ±20 m or 60 ft for altimeters with a test range of 0 to 9 000 m (0 to 30 000 ft); and
b) ±25 m or 80 ft for altimeters with a test range of 0 to 15 000 m (0 to 50 000 ft).
I suspect that the question you quote originates from the UK CAA in pre-EASA days. Another thread
here