Answer:
Bartering is the correct answer.
Explanation:
Kai is the engineer that setup his company of IT and his friend who sells the goods for his company. Then he decided to develop a software of the stock management for his friend's store for this his friend gives the required office goods and equipment for his company. So, the bartering is best suited for the following statement.
Public key and private key - In public key encryption, a pair of keys is used (public key and private key). The public key can be made available publicly, while the private key is only known by the owner. The public key is used to encrypt the message, while the private key is used to decrypt the message.