What is the most common element (by mass) forming the planet earth?
A.Oxygen
B.Carbon
C.Iron
D.Silicon

Hint: Chemical elements most commonly occur in the earth’s crust and atmosphere. Earth is a planet consisting of so many chemical elements in the earth’s crust. Some of them are iron, and silicon. In the atmosphere gases will be present in higher quantities.

Complete answer: Earth is one of the planets in the solar system which is the only planet for the survival and existence of several organisms like humans and other animals. Earth has three layers like core, crust and mantle. The earth’s crust consists of chemical elements like iron and silicon, which are metals.
The atmosphere of the earth consists of gases like nitrogen, oxygen, hydrogen, helium and argon. The most common element in the earth’s atmosphere is nitrogen and followed by oxygen.
The most common element in the earth’s crust is iron. It will be available in the deeper layers of earth. Thus, it is the most common element by mass forming the planet earth.
Iron is the element with atomic number \[26\] and is the most common element occurring in the earth’s crust.
Hence the correct option is C.

Note:
The iron is the fourth most abundant element with \[6\% \] in the earth’s crust. The abundance of chemical elements were oxygen, silicon, aluminum and iron. But iron is the most common element by mass forming the planet earth which is nearly \[35\% \] which is followed by oxygen.