Answer:
c. oxygen
Step-by-step explanation:
Oxygen -
The most abundant element of the Earth's crust is the oxygen , which constitutes to 46.6 % .
Oxygen is mainly found in the combined form as , phosphates , carbonates .
It has many application in the field of medicine , industries and commercial purpose .
In hospital it is used for the treatment of respiratory diseases .
Hence , the most abundant element on planet earth is c. oxygen .