When HBr (one equivalent) adds to butadiene at 40 °C, the major product is

When HBr (one equivalent) adds to butadiene at 40 °C, the major product is ${\mathrm{BrCH}}_2\mathrm{CH}={\mathrm{CHCH}}_3$.

The kinetic product is the 1, 2-addition product under moderate conditions (temperature -80°C), while the thermodynamic product is the 1, 4-addition product under vigorous conditions (temperature 40°C). As a result, the main product in the scenario is 1-bromo-2-butene.

Hence, option B is correct.