Final answer:
Scientists used radioactive decay (radiometric dating) to determine the age of Earth, which is at least 4.3 billion years old. This method measures the decay of radioactive isotopes in rocks to establish their age.
Step-by-step explanation:
The method that allowed scientists to determine that Earth was at least 4.3 billion years old is B) Radioactive decay. This process, known as radiometric dating, involves measuring the decay of radioactive isotopes in rocks and fossils to determine their ages. While carbon dating is a well-known form of radiometric dating, it is mostly applicable for dating biological tissues up to about 50 or 60 thousand years old, not billions. Therefore, other isotopes with longer half-lives are used for dating very old rocks. When using radiometric dating, scientists measure the ratio of 'parent' isotopes to 'daughter' isotopes in a sample. This ratio allows them to calculate the amount of time that has passed since the rock or fossil formed.
For example, measuring the age of rocks can provide approximate dates for the formation of the Earth and evidence of the earliest life forms based on the fossils contained within them. As radiometric dating reveals, some of the oldest rocks on Earth are found to be around 3.5 billion years old, and thus, indicate that the Earth itself must be even older.