Which of the following is a study within bioastronautics?

Bioastronautics is an interdisciplinary field that focuses on the biological and physiological challenges that humans face when living and working in space. It particularly emphasizes how space conditions, such as microgravity, affect human health and performance. Therefore, the study of the effects of microgravity on human health is a key area within bioastronautics.

This includes researching physiological adaptations, potential health risks, psychological factors, and countermeasures necessary for astronauts during space missions. Such studies help in developing strategies to maintain health and performance in long-duration space missions, making it fundamental to the field's objectives.

The other choices, while related to space and science, do not fall under bioastronautics. Astrophysics deals primarily with the physics of celestial bodies; spacecraft engineering focuses on the design and construction of space vehicles; and planetary geology studies the composition and processes of planetary bodies. Each of these fields has a separate focus and does not directly address the health and physiological challenges faced by humans in space.