Which one of the following non-metals is not a poor conductor of electricity?

The correct answer is Selenium. In general, non-metals are poor conductors of electricity because they do not have free electrons to carry a charge. However, Selenium is an exception to this rule due to its unique property of photoconductivity. Its electrical conductivity increases significantly when it is exposed to light. While Sulphur, Bromine, and Phosphorus are typical non-metals that act as insulators or very poor conductors, Selenium is a semiconductor. This means it can conduct electricity under certain conditions, making it the only option in the list that is not a poor conductor. This property makes Selenium highly useful in devices like solar cells and light sensors.