Final answer:
Among the options provided, charcoal is opaque because it does not allow light to pass through, preventing one from seeing through it.
Step-by-step explanation:
An opaque object is one that does not allow any light to pass through; it does not transmit light. In contrast, transparent objects allow light to pass through clearly, and translucent objects allow some light through but scatter it, causing objects behind them to appear blurred. Among the options provided in the student's question, A) Charcoal is an opaque material. Light does not pass through charcoal, making it impossible to see through. This is because charcoal absorbs light rather than allowing it to pass through or scatter. In contrast, B) Air and D) Water are transparent, while C) Glass is usually transparent unless it has been treated to be opaque or translucent.