- Mesophilic digestion occurs at temperatures around body temperature (~37°C). It is a slower but more stable process than thermophilic digestion. - Thermophilic digestion occurs at higher temperatures, typically 50°C to 57°C, and is faster but less stable and requires more energy input. - Option C (37°C) falls within the optimal mesophilic range. Option D (50°C) falls within the thermophilic range. Without specifying the type, 37°C is a very common operational temperature for conventional anaerobic digestion plants.