Correct Answer: Regulate

Solution : The correct choice is the third option.

Disorder refers to a lack of order or a state of confusion. Regulate means to organise according to a set of rules or standards. In the given sentence, the speaker is expressing a lack of order or organisation in their bookshelves, making regulate the suitable antonym for disorder in this context.

The meanings of the other options are as follows:

• Condition: It relates to the state or quality of something.
• Chaos: It refers to a state of disorder and confusion.
• Plague: It can either refer to a widespread disease or more broadly any widespread affliction or trouble.