ANS:C - dilute solution and isothermal operation.

In an absorber, the equilibrium line and operating line represent the relationship between the concentrations of the solute in the liquid phase (typically the solvent) and the gas phase (typically the absorbate). The shape of these lines depends on various factors such as the concentration of the solution, the temperature, and the nature of the solute-solvent system. Let's analyze the given options:

1. Concentrated solution and non-isothermal operation: In this scenario, both the equilibrium and operating lines are not expected to be straight. Concentrated solutions can lead to non-linear equilibrium relationships, and non-isothermal operation introduces temperature variations, which can further complicate the behavior of the system.
2. Dilute solution and non-isothermal operation: Similar to the previous scenario, non-isothermal operation introduces temperature variations, which can lead to non-linear behavior. Dilute solutions may exhibit more linear behavior compared to concentrated solutions, but the combination of non-isothermal operation may still result in curved lines.
3. Dilute solution and isothermal operation: Dilute solutions generally exhibit more linear equilibrium relationships, and isothermal operation ensures constant temperature, which simplifies the behavior of the system. In this case, both the equilibrium and operating lines are more likely to be straight.
4. Concentrated solution and isothermal operation: Concentrated solutions may result in non-linear equilibrium relationships, while isothermal operation ensures constant temperature. This combination may result in curved equilibrium and operating lines.