Saprotrophs are organisms that obtain their nutrition by feeding on dead and decaying organic matter. They are also known as decomposers or detritivores. The correct answer to the given question is option 'D', which states that saprotrophs are osmotrophs.

Explanation:

Osmotrophs are organisms that obtain their nutrition by absorbing dissolved organic compounds or particles through their cell membranes. Saprotrophs are osmotrophs because they absorb the nutrients from the dead and decaying organic matter through their cell membranes.

Saprotrophs play a vital role in the ecosystem by breaking down the complex organic matter into simple compounds that can be used by other organisms. They are responsible for the recycling of nutrients in the ecosystem. Some examples of saprotrophs are fungi, bacteria, and some protozoa.

Heterotrophs, on the other hand, are organisms that obtain their nutrition by consuming other living organisms or dead organic matter. Autotrophs are organisms that can produce their own food through photosynthesis or chemosynthesis. Phagotrophs are organisms that obtain their nutrition by engulfing other organisms or particles.

Conclusion:

Saprotrophs are osmotrophs that obtain their nutrition by absorbing dissolved organic compounds or particles through their cell membranes. They play a crucial role in the ecosystem by breaking down the complex organic matter into simple compounds that can be used by other organisms.