Which of the following is prohibited for mechanical refrigerating systems and portions thereof?

The prohibition of placing mechanical refrigerating systems and their components in spaces containing electrical equipment is primarily a safety measure. Electrical equipment poses a risk of sparks or overheating, which can ignite flammable refrigerants or compressor oils commonly used in refrigerating systems. This creates a hazardous environment where the risks of fire or explosion significantly increase.

The other options may present concerns, but they do not encompass the same level of immediate danger. Spaces with wooden structures could be an issue under certain conditions, especially relating to fire safety and the spread of flames, but they are not outright prohibited. Spaces without ventilation may lead to the buildup of refrigerants, which can be harmful to health, but still don't have the same immediate ignition risk as electrical equipment. Spaces with open flames pose a known hazard but are typically regulated separately in terms of safety codes for different types of equipment. Thus, placing mechanical refrigerating systems near electrical equipment stands out as a clearly defined prohibition in industry standards to ensure safety and minimize the risk of fire hazards.