The correct answer is (b) Microbial metabolic secretions
To explain I would say: Microbial metabolic secretions are the one which can dissolve the heavy metals present in the soil by secreting a special type of enzyme or any organic acid like low molecular weight. Whereas, biogeochemical cycle is crucial for methanogens, the natural release of biogas in the environment is due to the activity of methanogens, hence, it can’t dissolve metals. Also, afforestation and deforestation cannot dissolve metals.