Step-by-step explanation:
Reactivity of non-metals depend on their ability to gain electrons. So, smaller is the size of a non-metal more readily it will attract electrons because then nucleus will be more closer to valence shell.
Hence, both bromine (Br) and astatine (At) belongs to same group. Size of bromine is smaller than the size of astatine.
Hence, Br is the non-metal which will be more reactive than At.