Nissl bodies are mainly composed of Proteins and lipids, Nucleic acids and SER, DNA and RNA, Free ribosomes and RER

Nissl body is also called as nissl substance. It is a large granular body present in neurons. Their main function is to synthesize and release proteins that are important for neuronal growth and regeneration of axons. It is made up of rough endoplasmic reticulum (RER) and free ribosomes. Thus, the correct answer is option C.