What part of a tree is often used to determine its age through the study of annual rings?

The trunk of a tree is the part primarily used to determine its age through the study of annual rings, also known as tree rings or growth rings. Each year, a tree adds a new layer of wood tissue, which forms a ring in the trunk. By examining these rings, which can be seen when the trunk is cross-sectioned, one can count the number of rings to ascertain the tree's age.

The characteristics of the rings can also provide insights into the growth conditions during each year. For example, a wider ring often indicates a year of favorable growth conditions (such as ample water and nutrients), while a narrower ring may reflect less favorable circumstances. This analysis can also reveal information about the tree's life history and the environmental factors it has experienced.

In contrast, the other options do not provide the same clear indication of age. Roots mainly serve the function of nutrient and water uptake and do not develop rings in a manner that reveals age. Bark, while protective and essential for the tree, does not display annual growth rings—it serves a different purpose in the growth and survival of the tree. Although branches can show growth patterns, they do not represent a complete annual record of the tree's growth in the way trunk rings do.