How does a halogen react in an electrophilic substitution reaction
A is correct because the question specifically says an electrophilic substitution reaction (D will also react, but it's an electrophilic addition reaction). However, you also need a Lewis acid catalyst like FeCl$_3$. This is because, while the delocalised system of electrons in benzene is indeed electron rich, the fact that it's aromatic (there is a system of pi electrons delocalised throughout the ring) makes it very stable.
The mechanism of the bromination of benzene is shown below (image from Wikipedia as I don't have ChemDraw on this computer). The mechanism is the same for Cl$_2$.