explanation containing three of following points
M1 bromine and chlorine react by gaining electron/forming 1- or negative ion
M2 bromine atom larger (than chlorine atom)
M3 bromine (atom) has smaller/weaker attraction (from nucleus) for (outer shell) electrons (than chlorine) OWTTE
M4 so (bromine has) less tendency to gain electron/form negative ion (so less reactive than chlorine) OWTTE
ALLOW bromine has larger atomic radius ALLOW bromine outer (electron) shell further from nucleus ALLOW bromine atom has more (electron) shells (than chlorine)
ALLOW reverse argument for chlorine in M2 M3 M4