Why B? And why not the others?

Correct answer: B. Sri Jayawardenepura Kotte

Sri Jayawardenepura Kotte, a suburb a few kilometres east of Colombo, was designated as the seat of Sri Lanka's parliament in 1982 to relieve congestion in the older colonial capital, and it now serves as the country's official administrative and legislative capital even though almost nobody outside Sri Lanka has heard of it. Colombo, meanwhile, remains the country's largest city, its main port, and its de facto commercial and executive capital, which is why outside references often casually call Colombo "the capital" even though that is not, strictly, its formal designation. Kandy is a historic hill city and former royal capital of the Kingdom of Kandy, famous for the Temple of the Tooth, but it has not been the national capital since Sri Lanka gained independence. Galle and Jaffna are both major historic port cities, in the south and north of the island respectively, but neither has ever served as the national capital.

Source: Britannica: Sri Jayawardenepura Kotte; Colombo entries; corroborated by CIA World Factbook, Sri Lanka entry (capital field)