Answer
Solution (a) has the greater buffer capacity.
Work Step by Step
The buffer with a greater capacity is the one with a larger quantity of moles of its acid and its base.
Since both solutions have the same volume: $(100ml)$, the concentration will determine the greater number of moles.
(a) has 0.30M of its base and 0.30M of its acid.
(b) has 0.10M of its base and 0.10M of its acid.
Therefore, the buffer with the greater capacity is the first one: $(a)$.
