Final answer:
Carbon-14 is the best isotope for dating early human settlements in North America after the last ice age, as it is appropriate for materials up to about 50,000 years old.
Step-by-step explanation:
The best isotope for dating early human settlements in North America after the last ice age is Carbon-14. This is because Carbon-14 dating is effective for determining the age of organic materials up to around 50,000 years old, which covers the period of early human settlements in North America after the last ice age. Other isotopes like Uranium-235/238, Potassium-40, and Rubidium-87 have much longer half-lives and are used for dating geological events and materials that are millions to billions of years old. For human settlements which are much more recent, Carbon-14 provides sufficient accuracy.