Microorganisms can be defined as the organisms which are seen with the aid of microscope. This category includes the bacteria, algae, fungi, protozoa and viruses. These microorganisms play an important role in the decomposition of the biological materials. The nutrients are also recycled by these microorganisms. the microorganisms bound in a soil serve this purpose. Moreover, the microorganisms, such as algae carry out the process of photosynthesis and make their own food.
Some of the microorganisms have complex relationships with different animals and plants both, which help in their survival.
Hence, the right options will be "carry out photosynthesis, decomposition, and nutrient recycling'.
No the sun is actually a very small star compared to a lot of others it just appears to be very big because of how close it is to our planet