Why B? And why not the others?

Correct answer: B. Brasília

Brasília is a capital conjured from empty savanna: on 21 April 1960 Brazil moved its government about 600 miles inland to a brand-new city, planned by Lúcio Costa with sweeping modernist buildings by Oscar Niemeyer, its street plan famously resembling an airplane. The idea of an interior capital had been floated since 1789 and was even written into the 1891 constitution, but construction only began in 1956 - the city went from scrub to seat of government in under four years. Rio de Janeiro is the classic trap, since it really was the capital until 1960; São Paulo is only Brazil's biggest city; and Salvador was the colonial capital, but that ended back in 1763.

Source: Britannica: Brasilia (1960 capital transfer, Costa/Niemeyer design)