The addition of HBr to 1-butene gives a mixture of products is according to Markovnikov’s rule. It states that, on addition of hydrogen halide(such as HBr) to an unsymmetrical alkene, the hydrogen atom in the hydrogen halide forms a bond with the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms (the rich get richer). Since, C- 1 have greater number of hydrogens than C-2 thus H of HBr will attack on C-1.

Therefore, A and B will form as major and C as minor product.