The answer is A, they are connected to all levels.
Decomposers in a food web, such as bacteria or fungi, mainly get their nutrient by decomposing dead organic matter to release carbon dioxide (when they respire)to the air for producers (which are mainly plants).
The following basic food chain (which forms food web when many of these are connected) may help you understand better:
Producer→primary consumer→secondary consumer→tertiary consumer
Since all levels in a food web is formed from organic matter (including plants and animals), decomposers feed on them to get their nutrient, so they're actually connect to all levels.
B is not correct because secondary consumers are the carnivores, which are animals that consume other animals for food and nutrient.
C is not correct because primary consumers are herbivores, which are animals that consume plants (or producers) for food.
D is not correct because autotrophs are organisms that make their own food by themselves, which are usually by photosynthesis (the process to make glucose from light energy). They're usually green plants, and the "producers" from a food web.
