The correct option is A Ribosomes
RER (rough endoplasmic reticulum) is an extensive organelle composed of greatly convoluted but flattish sealed sacs, which are contiguous with the nuclear membrane. It is called as 'rough' endoplasmic reticulum because it is studded on its outer surface (the surface in contact with the cytosol) with ribosomes. These are called as membrane bound ribosomes and are firmly attached to the outer cytosolic side of the ER. About 13 million ribosomes are present on the RER in the average liver.