The instrument based on the following principle is used for the measurement of oxygen only in the flue gases.

ANS:D - Paramagnetism

The instrument used for the measurement of oxygen only in flue gases is based on the principle of paramagnetism. Paramagnetic oxygen analyzers operate on the principle that oxygen molecules are paramagnetic, meaning they are weakly attracted to a magnetic field. In these analyzers, a sample of the flue gas is drawn into a measurement chamber where it is exposed to a magnetic field. Oxygen molecules in the gas are attracted to the magnetic field, causing a change in the gas's magnetic susceptibility. This change is detected and correlated with the oxygen concentration in the flue gas. Paramagnetic oxygen analyzers offer high accuracy and sensitivity for measuring oxygen concentrations in flue gases. They are commonly used in industrial applications such as combustion processes, exhaust gas monitoring, and environmental emissions monitoring. Therefore, the correct answer is Paramagnetism.