The Constitution was ratified by state's conventions in 1787, 1788, 1789, and 1790. The convention of the ninth state (New Hampshire) ratified on June 21, 1788.

However, the operation of the Constitution didn't commence until March 1789 [1] [2]. Prior to that commencement, government still operated under the Articles of Confederation. The question is poorly worded, if they're asking for when the federal government began operating under the Constitution; but it's also poorly worded if they're asking for when the ninth state ratified it.

[1] https://supreme.justia.com/cases/federal/us/18/420/ [2] https://tile.loc.gov/storage-services/service/ll/usrep/usrep...