What is a common sign of moderate dehydration in children?

B is the correct answer because irritability and dry mucous membranes are observable signs that can indicate moderate dehydration in children. In cases of moderate dehydration, children may begin to show behavioral changes such as increased irritability, which is a response to not having enough fluids. Dry mucous membranes, particularly in the mouth and inside the nose, occur due to the loss of fluid and can be assessed easily through physical examination.

Severe lethargy is typically associated with more severe dehydration and is not a hallmark of moderate dehydration. Excessive urination would not typically be present in a state of dehydration, as the body usually conserves fluids during such a condition. Weight gain is not relevant in the context of dehydration; rather, weight loss may occur as a result of fluid loss and should be monitored in a dehydrated child.