Final Answer:
a. The Binary Search Tree (BST) constructed by inserting the keys 15, 17, 10, 2, 19, 18, 14, 8, 7, 16, 3, 4, 11 in order is as follows:
```
15
/ \
10 17
/ \ \
2 14 19
/ / \ \
8 11 16 18
/
3
\
4
```
b. After copying, the resulting BST tree without the node with value 15 is as follows:
```
14
/ \
10 17
/ \ \
2 11 19
/ / \ \
8 14 16 18
/
3
\
4
```
c. After merging, the resulting BST tree without the node with value 15 is as follows:
```
14
/ \
10 17
/ \ \
2 11 19
/ / /
8 14 18
/
3
\
4
```
Step-by-step explanation:
a. The BST is constructed by inserting keys in the order specified. At each step, a key is compared with the current node, and based on the comparison, it is placed in the left or right subtree accordingly.
b. To delete the node with value 15 by copying, the node with the next larger value (16 in this case) is found and replaced by copying its value to the node being deleted. The tree is then adjusted accordingly.
c. To delete the node with value 15 by merging, the node is replaced by its in-order predecessor (14 in this case). The in-order predecessor is the maximum value node in the left subtree of the node being deleted. The tree is then adjusted accordingly, ensuring the BST properties are maintained.