as there is given for loopback testing then 3)127.1.100.1 is the right answer.
let me describe this in brief.
a. if u developed a software that is having client and server mechanism and u want test that on ur computer than u can put loopback address at the destination and check for it without having any internet connection.
b.loopback address is done to check ur tcp/ip protocol and that works till network layer only.In this... packet is send by the network layer and received by that network layer only of the host .to make software to believe that they are connected to internet without actual connection.this is used to check proper functioning of upper layers only.
c.this loopback testing is done at ur computer so this destination address never ever be able to leave ur computer or host.
answer 1) is wrong as by this u can only test the internet connectivity of ur computer and u cant do loopback testing through this.