When a frog eats a grasshopper, where does the energy transfer occur?

Plants are eaten by grasshoppers; hence the grasshopper is the primary consumer. A frog eats the grasshopper, so the frog becomes the secondary consumer. The energy transfer is from the primary consumer to the secondary consumer, i.e., from the grasshopper to the frog. So the food chain represents the plants as producers, grasshoppers as primary consumers and frogs as secondary consumers. 

​​Plants ➡ Grasshopper     ➡      Frog​ 

(Producer) (primary consumer)    (secondary consumer) 

Hence, the correct option is (c) Primary consumer to secondary consumer.