Grammar rule: after the word "all" used to mean "everything", English always uses the relative pronoun "that", never "which" or "who". This is one of the fixed rules tested again and again in usage papers.
Apply the rule to the blank: "All ___ glitters is not gold" becomes "All that glitters is not gold" once we plug in "that". Read it aloud and it sounds like the familiar saying everyone already knows.
Options A and B, "those" and "these", point at specific things rather than joining a clause, so they cannot replace a relative pronoun. Option D, "the", is only an article and carries no pronoun function at all.
That leaves option C, "That", as the only word that completes both the grammar and the proverb correctly.