What is the primary function of the 'Oxygen Sensor' (Lambda Sensor) in the exhaust system?

1. To measure exhaust oxygen content, allowing the ECU to adjust the air-fuel ratio dynamically
2. To inject oxygen into the catalytic converter to assist in oxidizing hydrocarbons
3. To filter carbon soot particles out of the exhaust gas stream
4. To measure the temperature of the exhaust gas to protect the turbocharger