ANS:C - dilute solutions and isothermal operation.

In absorption processes, the operating line and equilibrium line represent two key aspects of the system:

1. Operating Line: This line represents the relationship between the concentrations of the solute in the liquid phase (typically the solvent) and the gas phase (typically the absorbate) at various stages of the absorption process. It's often determined by the mass balance equation and the material balance for the solute.
2. Equilibrium Line: This line represents the equilibrium relationship between the solute concentrations in the liquid and gas phases at a given temperature and pressure. It's typically determined by the equilibrium data for the specific solute-solvent system.

Now, regarding the statement:

• Isothermal Operation: When the absorption process occurs at constant temperature (isothermal operation), the equilibrium relationship between the solute concentrations in the liquid and gas phases remains constant.
• Dilute Solutions: In dilute solutions, the concentrations of the solute in both the liquid and gas phases are low, which means that the equilibrium relationship between the phases is relatively linear over the range of concentrations involved.

When both isothermal operation and dilute solutions are present:

• The equilibrium relationship between the phases remains constant due to isothermal operation.
• In dilute solutions, the equilibrium relationship between the phases is relatively linear.

As a result, both the operating line and equilibrium line will be straight under these conditions. So, the correct statement is: dilute solutions and isothermal operation.