Gary doesn't like yard work but he notices that the leaves on his lawn are starting to break down. What process is occurring here?

The process is decomposition.

Decomposition is a process by which organic substances, like leaves, are broken down into simpler matter. A lot of different types of organisms, called the decomposers, will consume the organic substances (like dead plants, dead animals) and continue an essential part of the nutrient cycle. This is important for recycling the organic matter that occupies space in the biosphere.

I would guess decomposition.

Is this a multiple choice question?