Water always boils when its

ANS:C - saturated vapour pressure equals the external pressure on its surface.

Water boils when its saturated vapor pressure equals the external pressure on its surface. At sea level, atmospheric pressure is typically around 760 mmHg or 1 atm. When the temperature of water reaches 100°C at this pressure, its saturated vapor pressure also becomes 760 mmHg. At this point, the vapor pressure of the water molecules at the surface equals the atmospheric pressure, and boiling occurs. So, the statement "saturated vapor pressure equals the external pressure on its surface" accurately describes the condition under which water boils.