Which of the following is a toxic chemical that may be found in certain types of transformers?

Polychlorinated biphenyls (PCBs) are indeed toxic chemicals historically used in transformers and other electrical equipment as dielectric fluids. They were favored for their insulating properties and thermal stability. However, due to their environmental persistence and potential health hazards, the use of PCBs has been heavily regulated or banned in many countries. Exposure to PCBs can lead to serious health issues, including cancer, immune system suppression, and reproductive and developmental problems.

In contrast, fibrillation is a medical term referring to a rapid and irregular heartbeat, which is not related to transformer substances. Silicone is commonly used as an insulator and sealant but is not considered toxic. Sodium chloride, known as table salt, is not toxic in the context of electrical equipment and does not serve as a component in transformers. This background helps contextualize why PCBs are identified as a significant concern regarding toxicity in transformers.