What are the organisms called that physically and chemically break down complex dead organic remains?

Decomposers

The correct answer is (b) decomposers.

Decomposers are organisms that break down complex dead organic remains physically and chemically. They play a vital role in the process of decomposition, where they break down organic matter into simpler inorganic substances. This allows the release and recycling of nutrients back into the environment.

Scavengers (option a) are organisms that consume dead animals or plants, but they primarily feed on relatively fresh remains. They contribute to the decomposition process by consuming the dead matter, but they do not break down the organic remains into simpler substances as decomposers do.

Parasites (option d) are organisms that live in or on another organism (the host) and derive nutrients from it. While parasites can cause harm or even death to their host, they do not play a significant role in the decomposition process.

Therefore, the correct choice is (b) decomposers, as they are specifically responsible for breaking down complex dead organic remains through physical and chemical processes.