Arrange the following alkenes in the descending order of their reactivity with $\mathrm{HBr}$.

a) Ethene     b) Propene     c) 2-Butene      d) 2-Methyl-2-butene

• The addition of $\mathrm{HBr}$ to alkenes takes places via ionic intermediates in Markovnikov addition sequence. 
• The alkene also produces ions (cation and anion), the more is the stability of carbocation more will be its reactivity towards $\mathrm{HBr}$. 
• More the number of methyl groups attached to alkene carbon more will be its stability. 
• The stability order of carbocation is 2-Methyl-2-butene>2-Butene>Propene> Ethene and so the same order follows towards reactivity with $\mathrm{HBr}$. 

Hence the correct option is (B).