The correct answer is C. Some chalk.

1. Chalks: This option is incorrect because "chalk" is an uncountable noun, and it is typically not used in the plural form when referring to the substance. The sentence requires an appropriate expression for uncountable nouns.

2. A chalk: This option is incorrect because "chalk" is usually referred to as a mass noun and not typically used with the indefinite article "a" when referring to the material in general.

3. Some chalk: This option is correct because "chalk" is an uncountable noun and using "some" correctly indicates an unspecified amount of it. This fits the context of needing an unspecified quantity of chalk for writing on the chalkboard.

4. Many chalks: This option is incorrect because "chalk" is not typically counted in terms of "many" but rather measured in terms of quantity. The plural form "chalks" is not standard for this context.

Summary:

- C. Some chalk is the correct choice because it appropriately uses the uncountable noun "chalk" with "some" to indicate the required quantity for writing on the chalkboard.