The correct answer is option A) Nucleotides.

Nucleic acids are biological macromolecules that are essential for the storage, transmission, and expression of genetic information. They are composed of monomers called nucleotides, which are the building blocks of nucleic acids.

Nucleotides consist of three main components:
1. Nitrogenous Base: This is the key structural component of a nucleotide and can be classified into two types: purines and pyrimidines. Purines include adenine (A) and guanine (G), while pyrimidines include cytosine (C), thymine (T) (in DNA), and uracil (U) (in RNA). The nitrogenous base is responsible for the specificity and complementary base pairing in nucleic acids.
2. Pentose Sugar: Nucleotides also contain a five-carbon sugar known as a pentose. In DNA, the pentose sugar is deoxyribose, while in RNA, it is ribose. The pentose sugar provides the backbone of the nucleic acid chain and connects the nitrogenous bases.
3. Phosphate Group: The phosphate group is attached to the pentose sugar and plays a crucial role in the structure and function of nucleotides. It provides a negative charge and allows for the formation of phosphodiester bonds between nucleotides, creating a polymer chain.

Nucleotides can exist as free nucleotides or can be linked together through phosphodiester bonds to form nucleic acids. DNA (deoxyribonucleic acid) is composed of deoxyribonucleotides, while RNA (ribonucleic acid) is composed of ribonucleotides.

Nucleosides, on the other hand, are similar to nucleotides but lack the phosphate group. They consist of a nitrogenous base bonded to a pentose sugar. Nucleosides are formed when the phosphate group is removed from a nucleotide. While nucleosides have important roles in cellular metabolism and signaling, they are not considered the building blocks of nucleic acids.

Amino acids are the building blocks of proteins, not nucleic acids. Proteins are a different class of macromolecules that perform a wide range of functions in cells.

Histones are proteins that help organize and package DNA within the nucleus of eukaryotic cells. They are not the building blocks of nucleic acids but rather assist in the formation of chromatin structure.

In summary, nucleotides are the building blocks of nucleic acids. They consist of a nitrogenous base, pentose sugar, and phosphate group, and are linked together through phosphodiester bonds to form DNA and RNA.

Deoxyribonucleoside triphosphate or nucleotides