Every _____, 15 million blood cells are destroyed in the human body.

The correct choice is "second" because, on average, the human body continuously produces and destroys blood cells to maintain homeostasis and overall health. Each second, approximately 15 million blood cells, particularly red blood cells, are destroyed. This high turnover rate is essential due to the finite lifespan of blood cells, especially red blood cells, which is around 120 days. The body needs to constantly generate new cells in the bone marrow to replace those that are lost through natural processes, such as aging and mechanical wear and tear.

This rapid destruction and subsequent production cycle highlights the efficiency and adaptability of the human body's systems in maintaining sufficient oxygen transport and overall circulatory function. The other choices—minute, hour, and day—do not align with the actual rate of blood cell destruction, making them incorrect in this context.