Which could be in the fourth trophic level of a food chain?
a. earthworm
b. grass
c. bluebird
d. rabbit

c. bluebird

Food chain refers to a process cycle from producer to consumer to decomposer.
Plants – herbivores – omnivores – carnivores Start -> Plants -> herbivore insect -> spiders -> small bird -> Snake -> hawk -> fox -> lion -> when died the body will be eaten by hyena-> when hyena died -> body get decompose and be eaten  by worms and the rest will be plant’s fertilizer.

The process of the food chain will then again takes place over and over again. 

That is true! many fish species traveled to the u.s. because of it.