During an electrophilic addition reactions double bonds donate lone pair electrons to an electrophile.

Markovnikov's Rule: The addition of a proton acid to the double bond of an alkene results in a product with the acid hydrogen bound to the carbon atom that already has the greater number of hydrogen attached.

In the given reaction electrophile ${\mathrm{D}}^{+}$ adds to the double bond, a carbocation is formed. Then ${\mathrm{Cl}}^{-}$ is added at the electron deficient carbocation.