buffer question please help

When 1.00 mol dm–3 solutions of salicylic acid and sodium hydroxide are mixed a buffer solution can be formed. Salicylic acid is a monoprotic acid that can be represented by the formula HA.
(a) Select a mixture from the table below that would produce a buffer solution.

Mixture vol. salicylic acid/cm3 vol. sodium hydroxide/cm3
X 25 75
Y 50 50
Z 75 25

The mark scheme states the answer as 'Z' with the reason being 'The idea that the solution contains both HA and A−'
Can someone please explain I don't understand this answer

When you mix the two solutions the salicylic acid and the sodium hydroxide will react togethor. You should (always) start by writing the balanced equation for the reaction. You will see that the acid and hydroxide react 1:1.

The reaction will form sodium salicylate, which provides the salicylate ion required in the buffer. The salicylate ion is the A^-.

The reaction will also 'use up' some or all of the acid and the hydroxide.
For each option X, Y and Z work out the number of moles of acid and alkali that you started with, and how much of the acid and alkali remain after they react. You will see that only option Z has excess acid, and therfore has some HA remaining to form a buffer. X and Y use up the acid completely so there is no acid HA to allow the solution to behave as a buffer.

The "trick" with a question like this is spotting that an acid and alkali are being mixed, realising that they will react to form the salt of the acid, and calculating how much acid will remain after that reaction has happened.