Final answer:
The most likely causal relationship is a positive correlation between time spent exercising and the number of calories burned, as it has a well-established scientific basis.
Step-by-step explanation:
Out of the given options, the relationship that would most likely be causal is (a) A positive correlation between time spent exercising and the number of calories burned. This is because there is a direct mechanism by which the body expends energy through physical activity, which results in calorie burn. This causal link has been well-established through various scientific studies. In contrast, options (b) and (c) show correlations that might not necessarily be causal, as they could be influenced by various other factors. Option (b) could have underlying factors such as overall time management or preferences that influence both activities, and option (c) may be explained by lifestyle patterns rather than a direct cause-and-effect relationship.