Final answer:
An object with a density of 0.9 g/cm³ will float in water, as its density is less than the density of water (1.0 g/cm³).
Step-by-step explanation:
If an object has a density of 0.9 g/cm³ and you put it into water, it will float. The density of water is 1.0 g/cm³, and since the object's density is less than the density of water, it will experience a buoyant force that is strong enough to keep it afloat. According to Archimedes' principle, a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid displaced by the body. Hence, if the object's density is less than the fluid's density, it will float.