By Which Material is a Transformer Body Made of?

If you mean core of transformer, there are different material s for different type of
transformers,for example:

1.small and big power and distribution transformers have core made of laminations or sheets
of alloy made of Iron ,cobalt and nickel.This lamination are used because the alignment of
electron spins in a ferromagnetic crystal tend to be along certain preferred directions of easy
magnetization.,this alignment is expected to cause every grain of iron alloy to be a permanent
magnet.The hysteresis loss is also reduced.Basically these are called magnetically soft alloys .

2.Manganese, zinc, ferrous cubic ferrite cube is used as low Eddy current loss iron alloy in
applications from low frequencies to microwave applications.

3. Winding: Copper
Insulation: Cellulose based paper and pressboard; transformer oil.
Tank: Carbon steel (mild steel).