Concept:After the word "all", the correct relative pronoun is "that".
Explanation:The blank introduces a relative clause that describes "all".
In standard English, we say "all that was said", not "all what" or "all which".
"Which" is not used after "all" in this pattern.
"Who" refers to people, so it is wrong here.
"what" means "the thing that", so it cannot follow "all".
Therefore, the best completion is "that".
Answer:A. that