The correct answer is (d), Sonbhadra.
Allahabad district is located in the state of Uttar Pradesh, India. It is bordered by the districts of Kaushambi, Fatehpur, Pratapgarh, Rae Bareli, Unnao, Banda, Mirzapur, and Sonbhadra.
Chitrakut district is located in the state of Uttar Pradesh, India. It is bordered by the districts of Mahoba, Banda, Hamirpur, and Satna.
Jaunpur district is located in the state of Uttar Pradesh, India. It is bordered by the districts of Azamgarh, Mau, Ghazipur, Chandauli, Mirzapur, and Sant Ravidas Nagar.
Sant Ravidas Nagar district is located in the state of Uttar Pradesh, India. It is bordered by the districts of Allahabad, Mirzapur, Chandauli, Varanasi, and Jaunpur.
Sonbhadra district is located in the state of Uttar Pradesh, India. It is bordered by the districts of Mirzapur, Chandauli, Varanasi, Bhadohi, and Rewa.
Therefore, the only district that does not form a border with Allahabad district is Sonbhadra.