ANS:C - moisture

Chlorine acts as a bleaching agent only in the presence of moisture. Chlorine gas or chlorine-containing compounds are often used as bleaching agents in various applications such as in the textile industry or in the treatment of drinking water. However, for chlorine to effectively bleach materials, moisture is required to facilitate the chemical reactions involved in the bleaching process. Moisture helps activate the chlorine and enhances its ability to react with and remove colorants or stains from the material being bleached. Therefore, chlorine's bleaching action is most effective when it is in the presence of moisture.