Introduction:
In computer science, a linear data structure is a data structure in which elements are arranged in a linear order. This means that the elements are stored in a sequence and each element has a unique predecessor and successor. Examples of linear data structures include arrays, linked lists, and queues.

Explanation:
Among the given options, the correct answer is option 'D', which states that a tree is not a linear data structure. Let's discuss why a tree is not considered a linear data structure.

Definition of a tree:
A tree is a hierarchical data structure that consists of nodes connected by edges. It is a non-linear data structure as it does not store elements in a linear sequence.

Key differences between a linear data structure and a tree:
1. Sequence: Linear data structures store elements in a specific sequence, where each element has a unique predecessor and successor. In contrast, a tree does not have a specific sequence as the nodes are connected in a hierarchical manner.

2. Connectivity: In linear data structures like arrays, linked lists, and queues, elements are connected in a linear fashion. However, in a tree, nodes are connected in a branching fashion, where each node can have multiple child nodes.

3. Hierarchy: Linear data structures do not have a hierarchical relationship between elements. On the other hand, a tree is a hierarchical structure where each node (except for the root) has a parent node and zero or more child nodes.

4. Traversal: Linear data structures are typically traversed sequentially, either from the beginning to the end or vice versa. In contrast, tree traversal involves various algorithms such as depth-first search or breadth-first search to visit nodes in a hierarchical manner.

Conclusion:
In summary, a tree is not considered a linear data structure because it does not store elements in a linear sequence. Instead, it organizes elements hierarchically through relationships between nodes. Linear data structures like arrays, linked lists, and queues have a specific sequence and connectivity between elements, which is not present in a tree.