What type of pronoun includes 'that', 'which', 'what', and 'who'?

The correct choice is relative pronoun. Relative pronouns are used to connect clauses or phrases to nouns, providing essential or additional information about those nouns. They introduce relative clauses, which modify a noun, adding useful details or clarifying context.

For example, "The book that you lent me was fascinating" uses "that" as a relative pronoun to link the clause "you lent me" to "the book," giving more information about which book is being discussed. Similarly, "who" is used to describe a person, while "which" typically refers to animals or things. These pronouns play a crucial role in sentence structure by helping to create more complex and informative sentences.