Which metal is the most abundant on earth and is rarely used for the main structural buildings because of production cost?

• A. Titanium
• B. Iron
• C. Copper
• D. Aluminum

Answer: (D)

Aluminum is the metal described here. It sits as the most abundant metal in the Earth's crust, but turning bauxite into usable aluminum is energy-intensive, which makes the material costly. For the main structural framework of large buildings, cost per load and ease of fabrication matter a lot, and steel (an iron-based material) generally offers a lower cost per strength. That combination—high abundance yet high production cost—explains why aluminum is rarely used for primary structural frames, even though its light weight and corrosion resistance make it attractive for non-structural parts, facades, or secondary components. The other metals don’t fit this pattern as well: they’re either not as abundant, or not as economical for main structural use.