Trophic levels are formed by:

1. A

   Organisms linked in food chain

   This option claims that trophic levels are formed by organisms linked in a food chain. Energy flows through these connected feeding positions from producers to various consumer levels. Therefore, this is the correct official key.

2. B

   Plants only

   This option claims that trophic levels are formed by plants only. While plants form the base as producers, higher trophic levels include herbivores and carnivores. Thus, this is not the key.

3. C

   Animals only

   This option claims that trophic levels are formed by animals only. This ignores the primary producers like green plants that capture solar energy at the very first trophic level. Thus, this is not the key.

4. D

   Carnivorous animals only

   This option claims that trophic levels are formed by carnivorous animals only. Carnivores occupy only the secondary and tertiary consumer levels, omitting producers and herbivores. Thus, this is not the key.

Summary. Official key is (a) Organisms linked in food chain. Trophic levels represent the hierarchical stages in a food chain, starting from producers and moving through various consumer levels. Options restricting this to plants, animals, or carnivores alone are factually incomplete. Energy transfer and feeding relationships collectively define these ecological tiers. Hence, option (a) is the only comprehensive and correct choice.