Which of the following halides ionises most readily to a carbocation (heterolysis of the C-Br bond)?

Ionisation of 7-bromocyclohepta-1,3,5-triene gives the tropylium cation, planar with 6 pi electrons (aromatic), so the bromide is almost ionic. 5-Bromocyclopenta-1,3-diene would give the antiaromatic cyclopentadienyl cation (4 pi electrons), bromocyclohexane gives a simple secondary cation, and bromocyclopropane gives a highly strained cation.