Problem related to Reflection on Wires in communication

Hallo All,

I need some description about the some Reflection of wires in communication. I have attached the Picture of My question Along with Answers for details so I shall be really grateful to you people if somebody spares his time to solve my problem.

My Questions:

1- Can someone explain me why we have used Wave resistance in first part but not in 2nd part?What is condition to use it.

2- Can someone explain me please how he solved Part 3 as I need concept of it and I am not being able to get the whole concept.

3- What is concept of solving this type of question.

I shall be grateful to you if someone help me out in it. Thanks a lot.

Please see pictures.
Question.