Which dye is used to detect leaks in refrigerants?

Bromothymol Blue is an appropriate choice for detecting leaks in refrigerants because it is a pH indicator that changes color in response to the acidity or basicity of a solution. In the context of refrigerants, if there is a leak, the bromothymol blue indicator can help to identify the presence of certain fluids that may alter pH levels, signaling that a leak is present.

The other options provided serve different purposes. Nessler's Solution is commonly used for detecting ammonium ions rather than for refrigerant leaks. Red litmus paper and Phenolphthalein are both pH indicators, but they are less specifically suited to detect the types of leaks typically encountered with refrigerants. Therefore, Bromothymol Blue is the most suitable choice among the provided options for this application.