The correct answer is option 3: growth rings. Growth rings in trees are used to determine their age. Each year, a tree grows a new ring around its trunk. These rings can be seen when a tree is cut or a core sample is taken. By counting the number of growth rings, scientists can determine the age of a tree. Each ring represents one year of growth. The width of the rings can also provide information about the tree`s health and growth patterns. Girth (option 1) and height (option 2) are not reliable indicators of a tree`s age as they can vary greatly depending on environmental factors and species. General appearance (option 4) also does not consistently indicate the age of a tree.