The dimensions of four wires of the same material are given below, in which wire the increase in length will be maximum when the same strain is applied.
$ Y = { F \over A} { L \over l} \Rightarrow l \alpha { l \over A } \alpha {L \over \pi d^2 } $ $ \therefore l \alpha { L \over d^2 } ( As F and Y are constant ) $ The ratio of $ { L \over d^2 } $ is maximum for case CD