Which of the following sources of information available to social workers for client assessments is usually the primary source?

A) ​Information obtained from client interviews
B) ​Direct observation of clients' nonverbal behavior
C) ​Collateral information from relatives and friends
D) ​Personal experiences of the social worker based on indirect interaction with clients

Information obtained from client interviews is typically the primary source of information for client assessments. This allows for the social worker to gather information directly from the client themselves and can provide a more accurate understanding of their experiences, needs, and goals. Other sources such as observation and collateral information can also be important, but may not provide as much depth or accuracy as information obtained directly from the client. Personal experiences of the social worker based on indirect interaction with clients would not be a reliable source of information for client assessments.