3) D


Mass of cylinder with 250cm³ = 170g
Then, the mass is increased to 470g when the object is submerged.
Mass of object = 470-170 = 300g.


When two objects are placed in the measuring cylinder, volume increases from 250 to 350.
Volume of two objects = 350-250 = 100cm³
Volume of 1 object = 100/2 = 50cm³
Density = Mass/Volume = (300/50) g/cm³