Final answer:
Walking upstairs puts the leg muscles under greater tension due to the force of gravity.
Step-by-step explanation:
The leg muscle is most likely under greater tension when walking upstairs (option A).
Walking upstairs requires more effort from the leg muscles as they have to work against the force of gravity to lift the body up. This increases the tension on the leg muscles, particularly the calf muscles such as the gastrocnemius and soleus.
Walking downstairs (option B) also requires some effort from the leg muscles, but the tension is not as high as when walking upstairs.
Walking on an even level (option C) requires the least amount of effort, so the tension on the leg muscles is lower compared to walking upstairs or downstairs.