Nissl's bodies are mainly composed of

A Nissl body, also known as Nissl substance and Nissl material, is a large granular body found in neurons (cyton and dendrites). These granules are of rough endoplasmic reticulum (RER) with rosettes of free ribosomes and are the site of protein synthesis. 

Nissl’s bodies, also known as Nissl granules, are primarily found in the cytoplasm of neurons. They are involved in protein synthesis, which is essential for the growth and maintenance of neurons. Nissl’s bodies are mainly composed of free ribosomes and the rough endoplasmic reticulum (RER).

Free ribosomes are responsible for synthesizing proteins that function within the cytoplasm of the cell, while the RER is involved in the synthesis of proteins that are secreted from the cell or incorporated into cellular membranes. The rough appearance of the endoplasmic reticulum is due to the ribosomes attached to it, and the combination of these structures forms Nissl bodies.

Therefore, the correct answer is D - Free ribosomes and RER, as these structures are essential components of Nissl’s bodies and play a vital role in protein synthesis within neurons.