The correct answer is option 3, "Both (a) and (b)". Eutrophication refers to the process where a water body becomes overly enriched with nutrients, particularly phosphorus and nitrogen. This excessive nutrient load stimulates the growth of algae and aquatic plants, leading to increased organic matter production. The presence of excess organic matter in the water promotes the growth of bacteria and other microorganisms, which in turn increases the biological oxygen demand (BOD) of the water. BOD is a measure of the amount of dissolved oxygen needed by microorganisms to breakdown organic matter in water. Therefore, eutrophication enhances both organic matter production and biological oxygen demand, ultimately resulting in ecological imbalances in the water body.