Which of the following levels is the most efficient at energy conversion?
a. producers
b. primary consumers
c. secondary consumers
d. tertiary consumers

The answer is "producers".

Producers (plants) have the most energy in a food chain and also they are most efficient at energy conversion. They give a creature more energy than a primary consumer or secondary consumer would.  

Producers are any sort of green plant. Green plants make their nourishment by taking daylight and utilizing the energy to make sugar. The plant utilizes this sugar, additionally called glucose to make numerous things, for example, wood, leaves, roots, and bark. Trees, for example, they forceful Oak, and the fabulous American Beech, are models of producers.

The correct answer would be a. producers.

Producers or autotrophs are the organisms which synthesize their own food through biochemical reactions. The most common way of production of food is photosynthesis.

They are present at the first trophic level of the food chain and convert solar energy into the chemical energy.

They do not depend on other organisms for their nutrition and energy demands which make them the most efficient at energy conversion.