Question

coating of clean iron nail with brown powder in most air option respiration resting combustion photosynthesis stomata

Answer 1

The rusting of an iron nail is due to its reaction with moisture in air and oxygen. 
Rust is also known as iron oxide. The process in which an iron nail gets coated with rust is called rusting. 
Hope this helps :-) 

Answer 2

Hi !

The answer is - Rusting.

When iron nails comes in contact with the atmosphere, rust forms on its surface,which is brown in colour.
Hence,due to the formation of rust,this process is known as rusting.