Attack of the alkene on bromine gives the bromonium ion, which is positive ion. Alkyl groups are electron donating and carbocation-stabilising groups.
D has 2 alkyl groups, C has one and A has none, but the B has a electron withdrawing nitro group. So, higher the stability of bromonium ion formed higher will be the rate of bromination reaction. So, the correct order of rate of reaction is D>C>A>B .