The osmotic pressure of a phenol solution in an organic solvent is deternined to be 20% less than expected, it is due to

$$\begin{gathered} Phenol in an organic solvent \\ 2A\underset{ }{\to }{A}_2\underset{ }{\to }dimenstion \\ \mathrm{\pi }=\mathrm{iCRT} \\ \mathrm{if} 20\% \mathrm{reduced}, \mathrm{left} 80\%. \\ \mathrm{for} 40\% \mathrm{dimension} \\ \mathrm{\alpha } = \frac{\mathrm{i} -1 }{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}-1} = 0.4=\frac{\mathrm{i}-1}{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}-1} \\ -0.2 = \mathrm{i}-1 \\ \mathrm{i} = 0.8 \\ \mathrm{So}, \mathrm{now} \boxed{\mathrm{\pi }=0.8\mathrm{CRT} } \end{gathered}$$

Hence, the correct option is: (C) phenol is 40% dimerised