a) The acidic character of alcohols is due to the polar nature
of  O–H bond. An electron-releasing group $(–C{H}_3,–C_2{H}_5)$ increases electron density on oxygen tending to decrease the polarity of O-H bond. This decreases the acid strength.
Acidic strength- Tertiary alcohol $<$ Secondary alcohol $<$ Primary alcohol
b) Alcohols have high boiling point due to hydrogen bonding. Aldehyde $\&$ ethers have high boiling points than alkanes due to polar-nature of C-O bond.
Boiling point order- n-Butane $<$ ethoxyethane $<$ pentanal $<$ pentan-1-ol

c) Relative ease of dehydration of alcohols- Primary $<$ Seconadary $<$ Tertiary. 
Greater the no. of alkyl groups, more stable is the carbocation due to positive inductive effect.

d) In phenol, presence of O-H group strongly activates the aromatic ring towards electrophilic substitution reaction. Chloro group is weakly deactivating due to electronegative chlorine but o $\&$ p-director.
Due to nitro group, p-nitrophenol is strongly deactivating towards electrophilic substitution reaction.