In CH3CH2OH ( ethanol ), the bond that undergoes heterolytic cleavage most readily is
C-C
C-O
C-H
O-H
Heterolytic fission takes place when two atoms differ considerably in their electronegativities.
Therefore O-H bond will undergo heterolytic fission most readily because of the considerable difference in electronegativities of O atoms and H atom, It has a maximum electronegativity difference from all the pairs given in the question.
Hence, the correct answer is Option(4)