Final answer:
A platelet count below 100,000/mm³ is considered a red flag to exercise.
Step-by-step explanation:
Platelet count is an important factor to consider before exercising, as it can indicate the risk of bleeding. A low platelet count increases the risk of excessive bleeding or bruising during physical activity. Generally, a platelet count below 100,000/mm³ is considered a red flag to exercise, making option B the correct answer. At this level, it is recommended to avoid activities with a high risk of injury or trauma, such as contact sports or heavy weightlifting.