Concept:After the indefinite pronoun "all," the correct relative pronoun is "that."
Explanation:The completed sentence is: "All ______ glitters is not gold."
In this proverb, "all" acts as an antecedent meaning "everything."
A relative clause is needed to describe what "all" refers to.
The pronoun "that" correctly introduces this restrictive relative clause: "All that glitters is not gold."
"What" is not used after "all" in this pattern.
"Which" is possible but less common and does not fit this fixed expression.
"Whatever" cannot function as a relative pronoun here.
Therefore, the best choice is "that."
Answer:C. that