Which of the following elements does not show a disproportionation tendency?
Hint: For disproportionation tendency element must show both positive and negative oxidation number.
Being the most electronegative element, F can only be reduced and hence it always shows an oxidation number of -1. Further, due to the absence of d-orbitals it cannot be oxidised and hence it does not show positive oxidation numbers.
In other words, F cannot be oxidised as well as reduced simultaneously and hence does not show disproportionation reactions.