Which statement best describes how buffers perform their function?
they contain water, which causes compounds to dissociate.
they contain substances that can react with either added acid or base
they have hydronium ions, which lower the ph
they have hydroxide ions which raise the ph.

They contain substances that can react with either added acid or base.

Explanation:

1) Compound also dissociates in buffer solution because of ions present(depend on pH of it) it doesn't mean it must have water.

2)Let any buffer have weak acid and it's salt with strong base. Then if we add acid then anion of salt will combine with H+ and form weak acid(which is very less ionisable). When we add base then it will react with weak acid and form salt and water(secondary product as in option A).

Here both doesn't affect pH.

3)They have hydronium which depend on degree of ionisation of acid or base in it but they also have hydroxide.

4)Same as in option 3 thus these last two doesn't completely justify.