How does a Manifold Absolute Pressure (MAP) sensor calculate the air density inside the engine intake tract?

1. By measuring the physical temperature of the intake manifold mounting bolts
2. By outputting a variable electrical signal based on the absolute pressure inside the intake manifold relative to a vacuum reference
3. By counting the intake valve opening and closing cycles per minute
4. By measuring the chemical composition of the air-fuel mixture before it enters the combustion chamber