___________ is the ratio of the density of an object to the density of water at a specified temperature and pressure.

• Specific gravity which is also called relative density is the ratio of the density of an object to the density of water at 4 degrees Celsius at normal temperature pressure.
  Specific Gravity = $\frac{\mathrm{Density} \mathrm{of} \mathrm{the} \mathrm{substance} }{\mathrm{Density} \mathrm{of} \mathrm{water} }$
• The term can also refer to the ratio of a gas's density to the density of dry air at standard temperature and pressure. 
• Based on its value, specific gravity can tell us whether an object will sink or float in water.

Therefore,  'Specific gravity is the correct response.