Question

# Phenol is less acidic thanA) p-nitrophenolB) EthanolC) CresolD) Benzyl alcohol

We know that phenol contains only OH groups. If it releases the  ${{\text{H}}^{\text{ + }}}$ ion, the conjugate base is phenoxide ion. In p-nitrophenol, after the releasing of  ${{\text{H}}^{\text{ + }}}$ ions, the negative charge on the phenoxide ion will be stabilized by strong –M effect of nitro group in the para position. So, the conjugate base is more stable than phenol. So, p-nitrophenol is more acidic than phenol.
Anything which stabilizes the conjugate base will increase the acidity. Therefore $p_{Ka}$ is also a measure of how stable the conjugate base is. Put another way, strong acids have weak conjugate bases, and vice versa.