Final answer:
Gilderoy Lockhart provided a signed note for Harry Potter to access "Moste Potente Potions" from the Restricted Section to brew Polyjuice Potion.
Step-by-step explanation:
The book that Professor Gilderoy Lockhart signed off for Harry Potter to retrieve from the Restricted Section of the library was B) "Moste Potente Potions". This event occurs in "Harry Potter and the Chamber of Secrets", the second book in the Harry Potter series by J.K. Rowling. The permission was needed as part of the plot to brew Polyjuice Potion, which is a complex and potentially dangerous potion that allows the drinker to assume the form of someone else.