Which of the following is an example of a two-phase heat transfer fluid?

Anhydrous ammonia is an example of a two-phase heat transfer fluid because it can exist in both vapor and liquid states under varying pressure and temperature conditions. This characteristic allows it to transfer heat effectively in refrigeration systems and other applications where changing phases is utilized for cooling or heating processes.

In heat transfer applications, two-phase fluids are particularly advantageous as they can absorb significant amounts of heat during the transition from liquid to gas (evaporation) and release heat when transitioning from gas to liquid (condensation). This ability to leverage the latent heat of phase changes makes anhydrous ammonia an efficient option for heat transfer.

The other choices listed are generally not classified as two-phase heat transfer fluids in the same way. For example, water and propylene glycol are primarily single-phase fluids under most operational conditions and do not provide the same phase change advantages in heat transfer systems. Castor oil is likewise a single-phase fluid and does not exhibit two-phase behavior relevant to heat transfer applications.