Which of the following correctly defines “Mother-of-pearl”?

Q. Which of the following correctly defines “Mother-of-pearl”?
Answer: A hard iridescent substance forming the inner layer of certain shells
Notes: Nacre also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up the outer coating of pearls. It is strong, resilient, and iridescent.