Final answer:
The best method to determine the exact age of a fossil is through radiometric dating, either using Carbon-14 for younger fossils or other isotopes like uranium for older fossils. Relative dating and deductive reasoning are helpful but cannot provide a precise numerical age.
Step-by-step explanation:
To best determine the age of a fossil in years, radiometric dating is often the most accurate method when the fossil contains suitable elements for dating. For fossils up to about 50,000 years old, Radiocarbon dating using Carbon-14 (14C) is effective as it measures the decay of carbon within organic materials. For older fossils or rocks, other isotopes such as uranium can be used for radiometric dating since they have longer half-lives. Relative dating can provide a sequence of events and can be helpful when radiometric dating is not possible, but it will not give a numerical age. Combining the age of the rock layer where the fossil was found with deductive reasoning, informed by relative dating methods, provides additional context but on its own doesn't offer a precise age. Therefore, while all methods have their uses, radiometric dating is typically the most precise way to determine the actual age of a fossil in years.