Detailed Explanation:

A food chain represents the linear flow of energy from producers to consumers in an ecosystem.

Diatoms are microscopic algae and act as primary producers through photosynthesis, forming the base of aquatic food chains.

Crustaceans (like krill, copepods) are primary consumers that feed on diatoms and other phytoplankton.

Herrings are secondary consumers (small fish) that feed on crustaceans and zooplankton.

The correct energy flow follows: Diatoms → Crustaceans → Herrings.