Compound A has one alkyl chain and one double bond because any other combinations of functional groups or multiple double bonds would either lead to multiple possible bromination products or a lower ratio of hydrogen atoms to carbon atoms.
compound C arises from the monobromination of Compound A, it will have the same carbon skeleton and double bond location as Compound A, but with a bromine atom replacing a hydrogen atom somewhere on the molecule. The specific location of the bromine depends on the exact structure of Compound A.
When Compound B reacts with a strong base, a mixture of two products (C and D) is formed. This suggests that Compound B can undergo two separate elimination reactions depending on where the proton abstraction occurs either from the carbon next to the double bond or the carbon next to the bromine.