Final answer:
A liter is equal to 1,000 cubic centimeters or 1 cubic decimeter, and a milliliter is equal to 1 cubic centimeter.
Step-by-step explanation:
A liter (L) is defined as the volume occupied by a cube that measures 10 cm (or 1 dm) on each side. Therefore, one liter is equal to 1,000 cubic centimeters (cm³) or 1 cubic decimeter (dm³). A milliliter (mL) is a smaller unit of volume commonly used and is equivalent to the volume of a cube that measures 1 cm on each side. Consequently, 1 milliliter is equivalent to 1 cubic centimeter (cm³).
Considering the provided options and the fact that 1 mL is equal to 1 cm³, the correct answer to the question of what a milliliter is would be option (a) 1 cubic centimeter.