Energy flow in an ecosystem is unidirectional.

Concept: The above-given statement is true

1. An ecosystem's energy flow is unidirectional, beginning with the producers at the first trophic level and ending with the decomposers (6th Trophic Level). 
2. The first and second laws of thermodynamics can be used to explain how energy moves through an ecosystem. Thermodynamics First Law: It asserts that although matter or energy cannot be created or destroyed, they can be transformed into new forms. Thermodynamics' Second Law: Some energy is wasted as heat or entropy during the transfer of energy through one trophic level to another. 
3. For practically all ecosystems, the sun is the only and most important source of energy. Less than 50% of incoming solar radiation is photosynthetically active radiation, or PAR; which means that the solar energy incident on the Earth's surface cannot be totally turned into energy by producers or plants. 
4. Additionally, plants are only able to capture 1–5% of incident light or 2–10% of the radiation that is photosynthetically active. 
5. Net primary productivity is between 0.8 and 4 percent of incident solar radiation or 1.6 and 8 percent of PAR because respiratory energy losses, which account for around 20 percent of total energy losses, also occur. 
6. Only 10% of the energy in one trophic level can be transmitted to the next, according to Lindeman's Law, commonly known as the 10 percent law.

Hence, option A is the correct answer.