Final answer:
Isotopic decay (C) is the quantity used to determine the absolute age of rocks and fossils through the method of radiometric dating.
Step-by-step explanation:
The quantity that helps determine the absolute age of rocks and fossils is isotopic decay (C). Geologists use a method known as radiometric dating to measure the decay of radioactive materials within the rocks to find an exact age in years. This technique takes advantage of the known rates of decay of radioactive elements like carbon-14, uranium-235, and potassium-40.
While relative dating techniques, such as the study of stratigraphic layers or the use of index fossils, can establish whether one fossil is older or younger than another, they cannot provide a precise numerical age. Therefore, radiometric dating is the tool used for determining the actual age of geological materials.