Plants prepare glucose through the process of photosynthesis. Option 1, respiration, is the process by which plants and animals break down glucose to release energy. It is the opposite of photosynthesis. Option 3, degradation, refers to the breakdown or decomposition of organic matter, which is not directly involved in glucose production. Option 4, mineral absorption, pertains to the uptake of essential nutrients from the soil by plant roots, which is necessary for plant growth and function but is not directly linked to glucose synthesis.

Photosynthesis is the process by which green plants, algae, and some bacteria convert sunlight, carbon dioxide, and water into glucose and oxygen. During photosynthesis, plants use the energy from sunlight to convert carbon dioxide and water into glucose, a simple sugar. The glucose produced during photosynthesis serves as an energy source for the plant and is used to build other organic molecules required for growth and metabolism.

Therefore, the correct answer is option 2, photosynthesis, as it is the process through which plants prepare glucose.