Concept:Use a relative pronoun after the word “all” to connect the clause.Explanation:The sentence needs a word that introduces the clause “my friend told me” and refers to “all.”In standard English, after “all,” the correct relative pronoun is “that.”“What” is not used after “all” because “what” already means “the thing which.”So the correct sentence is: “I have forgotten all that my friend told me.”Answer:B. that