The answer is D. Chalk : Blackboard.
In the other three pairs, the first word is a tool that is used to create something with the second word. A saw is used to cut wood, a pen is used to write on paper, and an author writes books. However, chalk is not used to create a blackboard. Instead, it is used to write on a blackboard.
Chalk is a soft, white limestone that is used for writing on blackboards. It is made by grinding limestone into a powder and then mixing it with water. The mixture is then formed into sticks or blocks. Chalk is easy to erase from blackboards, and it does not leave any residue.
Blackboards are made of a hard, smooth surface that is ideal for writing on with chalk. They are typically made of slate or porcelain enamel. Blackboards are often used in schools and offices.
I hope this helps! Let me know if you have any other questions.