Which of the following is NOT a primary focus of geophysics?

Geophysics primarily involves the study of physical processes and properties of the Earth, focusing on phenomena such as seismic waves, magnetic fields, gravity, and the Earth's internal structure. Each of the other options—magnetic fields, rock formations, and gravity measurements—are essential aspects of geophysical study, examining the Earth’s properties, materials, and the forces that act upon them.

Plant physiology, on the other hand, deals with the biological functions and processes of plants and does not fall within the realm of geophysics. Instead, it focuses on aspects such as photosynthesis, plant growth, and nutrient uptake, which are more aligned with biology rather than the physical sciences that geophysics encompasses. Thus, this option is distinctly outside the primary focus areas of geophysics.