Entropy is a measure of the disorder or randomness of a system. For a phase transition, such as from liquid to gas, entropy increases due to the increase in the freedom of motion of molecules in the gas phase. In the reaction:
\[
\text{Br}_2 (l) \to \text{Br}_2 (g)
\]
there is a phase transition from liquid to gas, which increases entropy.
However, when the phase transition is from gas to liquid:
\[
\text{Br}_2 (g) \to \text{Br}_2 (l)
\]
the molecules in the gas phase become more ordered when they condense into the liquid phase, which results in a decrease in entropy.
Thus, entropy decreases in the reaction \( B \).