Q1: 
1. Recursion
2. Pointer
3. Data Structure
4.Linked-list
5. Array
6. Tree
Q2:
Use of appropriate data structure enables a computer  system to perform its task more efficiently, by influencing the ability of  computer to store and retrieve data from any location in its memory.  Different kind of data structures are suited to different computer applications and tasks.
Q3:
applications of the trees:

1.Databases also uses tree data structures for indexing.

2.Domain Name Server(DNS) also uses tree structures.

applications of a graph:

1.Facebook’s Graph API uses the structure of Graphs.

2.Google’s Knowledge Graph also has to do something with Graph.

 Applications of a stack:

1.Syntaxes in languages are parsed using stacks.

2.It is used in many virtual machines like JVM.

applications of a queue:

1.To handle congestion in networking queue can be used.

2.Data packets in communication are arranged in queue format.