Final answer:
Researchers compare the relative positions of fossils and rock layers to determine when an organism lived, using the method of relative dating. If needed, absolute dating methods like isotope ratio comparisons can also provide specific dates.
Step-by-step explanation:
To determine when an organism lived, researchers may compare the relative positions of fossils and rock layers, which is a method known as relative dating. This approach establishes which fossils are older or younger by examining their positions within geological formations. The fossils in the lower layers are generally older since these layers would have been laid down earlier. This is a crucial part of stratigraphy and helps in reconstructing the sequence of events in Earth's history.
Additionally, absolute dating techniques can be used, which establish a more precise age in years. This might involve the comparison of isotope ratios within the fossil material or surrounding rocks, a method that can provide an actual date for when the organism was alive. While not explicitly listed in your options, DNA sequencing can also assist in understanding the evolutionary relationships among organisms and can supplement dating information by providing a molecular clock that estimates when species diverged from a common ancestor. However, for the direct comparison of when an organism lived in terms of geological time, the correct answer is c) Relative positions of fossils and rock layers.