Suprascapular aery, Subscapular aery, Posterior intercostal aery and Veebral Aery may all form channels for collateral ciculation in coarctation of aoa. The veebral aery is probably the least prominent pathway for collateral circulation amongst the options provided and hence may be accepted as the single best answer by exclusion. Ref: Moss and Adams' hea disease, 7th Edition, Page 990; Gray's Anatomy, 36th Edition, Page 710, 40th Edition, Page 984, 985; Radiology of Peripheral Vascular Diseases By Zeitler, Ammann, 2000, Page 324; Aoic Diseases : Clinical Diagnostic Imaging Atlas By Hutchison (Elsevier), 2008, Page 273.