Final answer:
The greenish-yellow gas described in the question that is used as a bleach, water disinfectant and is found in table salt is chlorine (Cl).
Step-by-step explanation:
The compound described as a highly reactive, greenish-yellow gas used as a bleach and water disinfectant, and a component of table salt, is chlorine (Cl). Chlorine is known for its strong bleaching and disinfecting properties. When dissolved in water, it forms hypochlorous acid, which is a very effective germicide. Its role in forming sodium chloride (NaCl), or table salt, occurs when chlorine reacts with sodium in a chemical reaction, producing a compound that is essential for human survival. Thus, the correct answer to the question is a. Chlorine (Cl).