A tree is defined by its structure, which includes a trunk, branches, leaves, and roots. While trees can have flowers and fruits, they are not always present. Roots, however, are essential for the tree's survival as they anchor it in the ground and absorb water and nutrients. Therefore, "root" is the only option that is always associated with a tree. 
Why other options are incorrect:
1. Flower: Not all trees have flowers. Some trees are coniferous and do not produce flowers.
2. Fruit: Similar to flowers, not all trees produce fruit. Some trees produce seeds directly from leaves or cones.
3. Leaves: While most trees have leaves, there are some exceptions like palm trees, which have fronds instead of leaves.