Explanation:

Let’s break it down:

- CAP: This stands for “Cover And Plinth,” which is an open, temporary method for storing grains—not metal, not even close.

- PUSA Bin: These are storage bins developed by IARI, New Delhi, but they’re actually made of galvanized iron or steel. Designed for safe storage of grains, especially on farms.

- PAU Bin: This one’s designed by Punjab Agricultural University, and yes, it’s a metallic iron structure specifically made for storing grains at farm level.

- Hapur Tekka: Think of this as a traditional, indigenous storage structure from the Hapur region. Not a metal unit, more “desi” and usually earthen.

- Hagevu: This is a traditional storage vessel—commonly bamboo-made, definitely not metal.

So the point is:

 Option 3: PAU Bin is the metal iron storage structure.