Final answer:
Kepler's Third Law states T² (the square of the orbital period) is proportional to r³ (the cube of the orbital radius). To find T² and r³, square the period and cube the radius respectively.
Step-by-step explanation:
To use Kepler's law to find the relation between the square of the orbital period (T²) and the cube of the orbital radius (r³), we refer to Kepler's Third Law, which states that the square of a planet's orbital period is directly proportional to the cube of the semi-major axis of its orbit. This can be represented mathematically as T² ≈ r³. Therefore, the correct way to find T² and r³ is by taking the square of the orbital period and the cube of the orbital radius respectively, which corresponds to option a).