Final answer:
Option (iii) Wolf, grass, snake, tiger is not a valid food chain because it includes multiple top predators and lacks a primary consumer, disrupting the natural energy flow of a proper food chain.
Step-by-step explanation:
A food chain illustrates a single pathway through which energy and nutrients flow in an ecosystem. The correct sequence of a food chain must always start with primary producers that obtain their energy from the sun or chemical sources and progress up through various consumers, which can be herbivores, carnivores, and sometimes omnivores. In a valid food chain, each organism feeds on the preceding organism in the chain.
Examining the options provided, (i) Grass(lion, rabbit, wolf), (ii) Plankton, man, fish, grasshopper, and (iv) Frog, snake, eagle, grass, grasshopper, all include producers such as grass and plankton and various levels of consumers, fitting the characteristics of a food chain. However, (iii) Wolf, grass, snake, tiger does not depict a legitimate food chain because it includes multiple top predators (wolf, tiger) but lacks a primary consumer that feeds on grass. Thus, this grouping does not represent a natural flow of energy through a proper food chain.