That was to those asking a sign of HIS messiahship/authority.
Resurrection would certainly be a fitting sign.
Coincidentally, it would also have been proof that the sadducees hadn’t been Berean enough in their understanding of future events.
They should have been looking for one who would resurrect, as the scriptures foretold.
Mark 13:22 and John 4:48 refer to deceptions done by false Christs and false prophets, while Acts 4:30, Acts 5:12, Acts 14:3, Romans 15:19 and Hebrews 2:4 each speaks of the miracles performed by the Apostles which are no more being performed.
Beyond that, the resurrection.