Astone of mass 12g and having a density of 3g/cm^3 is carefully lowered...

The stone will move an amount equal to its volume, so the new reading in the measuring cylinder will be the moved water by the stone plus the original 25 cm^3
We calculate the volume moved by the stone using the density and mass of the stone. The density d is defined as the mass m divided by the volume v, so we solve for the volume:
d = m/v
v = m/d
and substituting the data we are given:
v = 12g/3 g/cm^3
v = 4 cm^3
therefore the volume occupied by the stone is 4 cm^3 that added to the original 25 cm^3 will give us a new reading of 29 cm^3