Micro-organisms play a crucial role in the decomposition of dead plants. They break down the organic matter present in dead plants through a process called decay. During this process, micro-organisms release enzymes that break down complex organic compounds in dead plants into simpler substances. These simpler substances eventually transform into a dark, organic material known as humus. Humus is rich in nutrients and is added to the soil as a result of the decomposition process. It improves soil fertility and structure by increasing its ability to retain moisture and nutrients. Therefore, the correct answer is option 3 - humus. It is important to note that micro-organisms do not produce sand (option 1), mushrooms (option 2), or wood (option 4) through the decomposition of dead plants.