Introduction:
In this question, we are given different pairs of traversal sequences on a labeled binary tree and we need to identify which pairs uniquely identify a tree.

Analysis:
To uniquely identify a binary tree, we need to have at least one traversal sequence which visits each node exactly once. Let's analyze each given pair of traversal sequences:

(i) Preorder and Postorder:
- Preorder traversal visits the nodes in the order: Root-Left-Right.
- Postorder traversal visits the nodes in the order: Left-Right-Root.
- As both traversals start with the root node, we can determine the root node using either of them.
- However, these two traversals alone cannot uniquely identify a binary tree because there can be multiple binary trees with the same preorder and postorder sequences. Hence, this pair does not identify a tree uniquely.

(ii) Inorder and Postorder:
- Inorder traversal visits the nodes in the order: Left-Root-Right.
- Postorder traversal visits the nodes in the order: Left-Right-Root.
- In this pair, the root node can be determined using the postorder traversal.
- However, these two traversals alone cannot uniquely identify a binary tree because there can be multiple binary trees with the same inorder and postorder sequences. Hence, this pair does not identify a tree uniquely.

(iii) Preorder and Inorder:
- Preorder traversal visits the nodes in the order: Root-Left-Right.
- Inorder traversal visits the nodes in the order: Left-Root-Right.
- In this pair, the root node can be determined using the preorder traversal.
- These two traversals together uniquely identify a binary tree. This is because the first element in the preorder traversal is always the root node, and the elements to its left in the inorder traversal represent the left subtree, while the elements to its right represent the right subtree. Hence, this pair identifies a tree uniquely.

(iv) Level Order and Postorder:
- Level order traversal visits the nodes in the order of their levels, from left to right.
- Postorder traversal visits the nodes in the order: Left-Right-Root.
- In this pair, the root node can be determined using the postorder traversal.
- However, these two traversals alone cannot uniquely identify a binary tree because there can be multiple binary trees with the same level order and postorder sequences. Hence, this pair does not identify a tree uniquely.

Conclusion:
Based on the analysis, the pairs of traversals that uniquely identify a binary tree are:
- Preorder and Inorder (option B).