Histone proteins not only keep the DNA in an organized form, but also regulate the gene expression. Every core histone protein has a tail, called the N-terminal tail. There are five major families of histones: H1/H5, H2A, H2B, H3 and H4. Among them H2A, H2B, H3 and H4 are the core histones, while H1 and H5 are linker histones. DNA is wrapped around an octamer of histone proteins, and forms the nucleosomes. Thus, the histone proteins are the building blocks of nucleosomes. The sequence of histone proteins is highly conserved in all eukaryotes.
Hence, the correct option is (c) a and b are correct.
Since the correct a and b are correct options, the option (d) is incorrect.