What is the correct way of using the aircraft call sign at first contact?

Question

Question Quiz BPL Hot Air - Balloon Pilot License - Communication

QuizVDS · Accepted Answer

Using all characters

QuizVDS · Answer

Using the last two characters only

QuizVDS · Answer

Using the first two characters only

QuizVDS · Answer

Using the first three characters only

Id 131 - BPL Hot Air - What is the correct way of using the aircraft call sign at first contact?