Micro – organisms act upon the dead plants to produce (i) sand (ii) mushrooms (iii) humus (iv) wood

Micro-organisms are decomposers that means thay decompose the organic waste.
When microorganisms act upon the dead plant ,i.e., organic waste, humus is produced.
Examples of microorganisms that act upon organic waste are – bacteria and fungi.
Final Answer :
Hence, the correct option is (iii), i.e. humus.