The standard air pressure at sea level is typically expressed in units of atmosphere (atm). Option 1 states that 1 atm is equal to 660 mm mercury, option 2 states 730 mm mercury, option 3 states 760 mm mercury, and option 4 states 780 mm mercury.

To determine the correct option, we need to recall the concept of atmospheric pressure. Atmospheric pressure is the force exerted by the weight of the Earth`s atmosphere on a given area. Mercury is commonly used to measure atmospheric pressure using a device called a mercury barometer.

The conversion factor between atmospheric pressure and mercury height is 1 atm = 760 mm mercury. This means that the correct answer is option 3, which states that 1 atm is equal to 760 mm mercury.

It is important to note that the provided answer is generally correct. However, depending on the context and the specific conditions, the standard atmospheric pressure at sea level can slightly vary. In practical usage, the value of 760 mm mercury is commonly used as a standard reference for atmospheric pressure.