The question is asking to identify the option that is NOT an alkali. An alkali is a base that dissolves in water to form a solution with a pH greater than 7. Therefore, we need to identify the option that is not a base or does not dissolve in water to form a solution with a pH greater than 7. Among the options provided, the only option that is NOT an alkali is NH3. NH3 is ammonia, which is a gas at room temperature and does not dissolve in water to form a solution with a pH greater than 7. Therefore, the correct answer is NH3.