Final answer:
To assess a client's activity level with a history of heart failure, it is most informative to ask if they can walk upstairs without fatigue, as this evaluates the impact of their condition on daily tasks.
Step-by-step explanation:
The nurse assessing a client with a history of heart failure would find it most helpful to ask if the client is able to walk upstairs without fatigue. This question directly assesses the client's activity level and provides insight into whether their daily activities are being limited by their condition. In heart failure, the pumping action of the heart is weakened, and simple activities like walking up stairs can become challenging and cause symptoms such as fatigue, shortness of breath, or chest pain. Thus, this question could be quite revealing about the patient's cardiovascular functional status.