You can find the ratio that it can expand with the atmospheric pressure.
200.0/101.3= 1.97 ≈ 2, this means that the balloon could expand twice its initial volume. So, it is not possible for the balloon to expand to 4 since the maximum is 2.
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, Br is the non-metal which will be more reactive than At.