Let's understand the meaning of the given words.
Colossus refers to a big statue.
Palace refers to a large and impressive building.
Tower refers to a long narrow building
Citadel refers to a tall fortress.
Castle refers to a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and in many cases a moat.
Column refers to an upright pillar, typically cylindrical, supporting an arch, entablature, or other structure or standing alone as a monument.
It is clear that "Colossus" refers to a huge statue.
Hence, option d is the correct answer.