The nucleus of a cell is most responsible for what function?

The main function of the cell nucleus is to control gene expression and to mediate DNA replication during the cell cycle. The nucleus provides a site for transcription in the cytoplasm, allowing levels of regulation that are not available in prokaryotes.

the answer is c) an oak tree

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.