The correct answer is: C. Iron-oxide
Iron-oxide is a constituent of good brick earth whose excess causes the raw bricks shrink and warp during drying and burning.
Alumina, lime, and magnesia are all essential constituents of good brick earth. They help to give the bricks strength and durability. However, iron-oxide can cause problems if it is present in too high a concentration.
When iron-oxide is heated, it expands. This expansion can cause the bricks to crack and warp. It can also make the bricks more susceptible to weathering.
For this reason, it is important to control the amount of iron-oxide in brick earth. The ideal concentration is between 2% and 5%. If the concentration is higher than this, the bricks may be more
likely to shrink and warp.If you are concerned about the iron-oxide content of your brick earth, you can have it tested by a professional. You can also try to reduce the iron-oxide content by adding other materials to the brick earth, such as sand or clay.