The answer is option 'A' i.e. 1 fluid remains at constant temperature during the process of boiling and condensation.

Explanation:

The process of boiling and condensation involves a change in the physical state of a substance from liquid to vapor and vice versa. During this process, the temperature of the substance remains constant until all the liquid has been converted into vapor or all the vapor has been condensed into liquid.

Boiling:

During the process of boiling, the liquid is heated to its boiling point, which is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. At this point, the liquid starts to boil and convert into vapor. During the process of boiling, the temperature remains constant until all the liquid has been converted into vapor. This is because the heat energy is being used to overcome the intermolecular forces between the liquid molecules and convert them into vapor.

Condensation:

During the process of condensation, the vapor is cooled down to its boiling point and then further cooled down to a temperature below the boiling point. At this point, the vapor starts to condense and convert into liquid. During the process of condensation, the temperature remains constant until all the vapor has been condensed into liquid. This is because the heat energy is being released by the vapor as it condenses and is used to overcome the intermolecular forces between the vapor molecules and convert them into liquid.

Conclusion:

Therefore, during the process of boiling and condensation, only one fluid remains at constant temperature i.e., either the liquid or the vapor, depending on the process being carried out.