Secondary alcohols will go cloudy in which of the following after gentle heating for 10 minutes?

Secondary alcohols will go cloudy when mixed with concentrated hydrochloric acid along with zinc chloride due to the formation of an intermediate complex that leads to the formation of alkyl chlorides. This reaction typically involves heating, which provides the necessary energy to facilitate the reaction.

In the presence of zinc chloride, which acts as a Lewis acid, the secondary alcohol can be converted to an alkyl halide through a series of steps: the alcohol first gets protonated to form a better leaving group and then reacts with the chloride ion to form the corresponding chlorinated product. The cloudiness arises from the formation of a colloidal suspension of the organic compounds as they convert, indicating that a reaction is occurring and that there is a product formation.

The other options do not provoke a similar reaction or result in a cloudiness. Water does not facilitate any significant reaction with secondary alcohols that leads to observable cloudiness; it typically remains clear as it interacts mildly with the alcohol. Concentrated HCl by itself might not show cloudiness without the zinc chloride, as there isn't enough catalytic action occurring to produce solid or colloidal products. Zinc oxide is an inorganic compound that does not react with secondary alcohols to produce a cloudy suspension.