The correct answer is: D. all of the mentioned
A POS (point of sale) is a system that allows a merchant to accept payment from customers. A QoS (quality of service) is a set of guarantees that a network provider makes to its customers about the performance of its network. A SoS (service level agreement) is a contract between a service provider and its customer that defines the level of service that the provider will deliver.
All of these things can be obtained under contract from a vendor. For example, a vendor might agree to provide a POS system to a merchant, and the contract would specify the features and functionality of the system, as well as the price and terms of the agreement. Similarly, a vendor might agree to provide a QoS guarantee to a customer, and the contract would specify the minimum level of performance that the provider will guarantee. Finally, a vendor might agree to provide a SoS to a customer, and the contract would specify the level of service that the provider will deliver, as well as the remedies that the customer will have if the provider fails to meet its obligations.