Leafy green vegetables such as spinach (Spinacia oleracea), lettuce, basil, and mint are considered ideal crops for aquaponic systems. They have low-to-medium nutrient requirements, short production cycles ($3$ to $5\text{ weeks}$), high commercial value, and do not require heavy support structures or root tuber development unlike fruiting trees or root crops.

Option analysis:

• Banana: Massive perennial herb, completely unsuited for standard hydroponic beds.
• Spinach: Correct, premier leafy green crop well-adapted to aquaponic nutrient film and gravel beds.
• Potato: Tuber crop requiring soil/dry mounding, prone to rot in aquaponic water media.
• Papaya: Large woody tree, unsuited for aquaponic systems.

The circular eco-hatchery (Chinese circular hatchery system) is the most widely adopted and efficient commercial hatchery design in India for large-scale seed production of Indian Major Carps. It features a circular spawning pool with tangential water jets that create a circular hydraulic current mimicking riverine flood conditions, coupled with a circular incubation pool providing continuous upward swirling aeration for millions of developing eggs.

Option analysis:

• Glass jar hatchery: Used for experimental or small-scale incubation (Bhowmick design).
• Vertical hatchery: Tray system used primarily for salmonid trout eggs.
• Circular hatchery: Correct, the dominant commercial eco-hatchery used throughout India.
• Hapa method: Traditional cloth enclosure method with low hatching survival and capacity.

In dry aquafeed storage, moulds (filamentous fungi such as Aspergillus flavus, Penicillium, and Fusarium species) represent the most frequent and economically devastating cause of spoilage. Whenever moisture content exceeds $10\%$ to $12\%$ in humid warehouses, fungal hyphae proliferate, degrading nutrients and synthesizing lethal mycotoxins (notably aflatoxin B1), causing severe hepatoma and mortality in farmed fish.

Option analysis:

• Bacteria: Require high water activity ($a_w \gt 0.90$), whereas dry feed has low moisture.
• Viruses: Obligate intracellular parasites, cannot grow or cause feed spoilage.
• Worms: Parasitic animals, not feed rot agents.
• Moulds: Correct, xerophilic and xerotolerant fungi that thrive on stored grains and feeds.