To identify which option does not represent a type of interaction among biotic factors in an ecosystem.

The question asks to identify which option does not represent a type of interaction among biotic factors in an ecosystem.

Let's examine each option:

• a. Parasitism: This is an interaction between two biotic factors (living organisms) where one benefits (parasite) at the expense of the other (host).
• b. Decomposition: This is the process where biotic factors (decomposers like bacteria and fungi) break down dead organic matter (which was once a biotic factor). This is an interaction involving biotic factors.
• c. Weathering: This is the process of breaking down rocks, soil, and minerals. While biological organisms can contribute to weathering (biological weathering), the term "weathering" primarily refers to physical and chemical processes driven by abiotic factors (like water, wind, ice, temperature changes) acting on geological materials. It is not primarily an interaction among biotic factors.
• d. Predation: This is an interaction between two biotic factors (living organisms) where one (predator) hunts and kills another (prey) for food.

Therefore, weathering is the option that does not primarily represent an interaction among biotic factors.

The final answer is $\boxed{\text{c. Weathering}}$