On se sertd' un/une.......pour écrire au tableau noir.

The question is asking about the tool used to write on a blackboard.

Let's examine the options one by one:

• Craie (Chalk): This is a soft, white, porous sedimentary carbonate rock. Chalk is typically used to write on blackboards because it easily rubs off to allow new writing.
• Crayon (Pencil): Pencils contain graphite and are typically used to write on paper. They are not suitable for blackboards as they don't write and erase well on such surfaces.
• Règle (Ruler): A ruler is used for measuring lengths and drawing straight lines. It is not a writing instrument.
• Plume (Pen): A pen uses ink and is suitable for writing on paper. It is not used on blackboards as it is permanent and cannot be easily erased.

Therefore, the correct answer is Craie (Chalk) because it is specifically designed for use on blackboards due to its ability to make visible marks easily and be wiped off without much effort.