This sentence is a well known proverb, and it needs a relative pronoun that can refer back to "all" and introduce the clause "glitters".
- Option A, "those": "Those" is a plural demonstrative pronoun and cannot act as a relative pronoun joining a clause here, so it is wrong.
- Option B, "these": "These" is also a demonstrative pronoun, not a relative pronoun, so it cannot link "all" with "glitters".
- Option C, "that": "That" is the relative pronoun regularly used after words like "all", "same", "only", and superlatives. It fits perfectly and gives us the correct proverb, "All that glitters is not gold."
- Option D, "the": "The" is an article, not a pronoun, so it cannot stand in place of a missing subject for "glitters".
The correct choice is option C, "That".