Final answer:
Among the side effects listed, the client should report abdominal pain immediately as it may indicate a serious condition like liver damage, which is an urgent concern when taking valproic acid.
Step-by-step explanation:
The medication valproic acid is an anticonvulsant used to manage seizures and is associated with various side effects. When providing instruction to a client who has seizures, it is critical to emphasize the importance of reporting certain adverse effects to the healthcare provider immediately. Among the options provided, it is vital for the client to report abdominal pain without delay, as it can be a sign of liver damage, which is a serious and potentially life-threatening adverse effect of valproic acid. While other side effects such as hair loss, weight gain, and ataxia can occur, they do not typically constitute medical emergencies like severe abdominal pain does.