Explain how does the –OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?
The -OH group is an electron-donating group. Thus, it increases the electron density in the benzene ring as shown in the given resonance structure of phenol. As a result, the benzene ring is activated towards electrophilic substitution.

Arrange the following compounds in increasing order of their boiling points:

The reaction 
suggests that phenol is :
In the reaction R–OH + HCl $\xrightarrow{\text{ZnCl}_2}$ RCl + H$_2$O, what is the correct order of reactivity of alcohols?

Alcohols, phenols, and ethers are organic compounds that can be classified based on their molecular structure and functional groups.
Classification of Alcohols:
Classification of Phenols:
Classification of Ethers:
In summary, alcohols, phenols, and ethers can be classified based on their molecular structure and functional groups. Understanding the classification of these compounds is important for predicting their reactivity and understanding their potential applications in various fields, including chemistry, biology, and industry.