The answer is c) both (a) and (b). An instant messaging service can accept instant messaging from both external sites and internal sites. External sites refer to websites or applications that are outside of the organization, while internal sites refer to websites or applications that are within the organization.
For example, an instant messaging service may allow users to chat with friends on other external instant messaging platforms, as well as allow employees within the same organization to communicate through the service. This can provide a convenient way for users to communicate with a wide range of people, both inside and outside of their organization.