Final answer:
The client omitting insulin doses when meals are missed is the best indication they are not coping well with diabetes management, as consistent insulin therapy is critical for controlling blood glucose levels.
Step-by-step explanation:
When a nurse is caring for a client diagnosed with diabetes mellitus, certain behaviors can indicate the client is not coping well with the disease. Among the options provided, the finding that best indicates a client is not coping with diabetes is that the client omits the insulin dose if a meal is missed. Skipping insulin doses can lead to hyperglycemia or other complications and is a sign that the client may not understand the importance of insulin therapy, or they may be struggling with the regimented nature of diabetes management. Other options, such as crying, monitoring blood glucose, and a modest weight gain, might not directly indicate poor coping, as crying can be a normal emotional response, monitoring glucose levels is a part of responsible diabetes care, and weight gain could be influenced by various factors.