Name the organelles that play a role in biosynthesis?

  I believe you want ribosomes which produce proteins from amino acids. Also the nucleus produces DNA from nucleotides and the nucleoli produce RNA from the same.

Biosynthesis in this sense is the building of larger molecules from smaller ones

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.

C) to ensure each new cell has an exact copy of the dna