The true statement among the options provided is:
Composting speeds up the decomposition of organic material.
This statement is correct because composting creates an environment that promotes the rapid breakdown of organic material by providing the right balance of moisture, aeration, and a mix of green (nitrogen-rich) and brown (carbon-rich) materials, which supports both microorganisms and invertebrates like insects that assist in decomposition.
The other statements are not true. Insects can actually aid decomposition but do not delay it; organic material can decompose naturally without composting; and decomposition involves many organisms, not just microorganisms.