Answer
Yes, (b) is the most basic solution.
Work Step by Step
As we have calculated in (a), this is the concentration for $2.0 \times 10^{-5}M$ $HBr$: $[OH^-] = 5 \times 10^{- 10}M$
The most basic solution is the one with the greatest hydroxide concentration.
Since (b) has $[OH^-] = 5 \times 10^{- 9}M$, its hydroxide concentration is greater, therefore, (b) is the most basic solution.
