Q.What type of linkages hold together monomers of DNA?

RNA's nitrogenous bases are adenine and guanine (purines) plus cytosine and uracil (pyrimidines) — uracil replaces DNA's thymine in RNA.

Of the five standard nitrogenous bases found in nucleic acids, two are purines (adenine, guanine — common to both DNA and RNA) and three are pyrimidines (cytosine, thymine, uracil). Cytosine is common to both DNA and RNA. The second pyrimidine differs: DNA uses thymine (5-methyluracil), while RNA uses uracil (thymine without the methyl group) in its place. So the pyrimidine bases specifically found in RNA are cytosine and uracil; thymine (option a and d, paired incorrectly with cytosine) belongs to DNA, not RNA, and adenine (option b) is a purine, not a pyrimidine.

DNA contains adenine, guanine, cytosine and thymine as its four bases; uracil is found in RNA (in place of thymine) and does not occur in DNA.

Nucleic acids use two purine bases (adenine, guanine) common to both DNA and RNA, and one pyrimidine base each that differs between the two: DNA uses thymine (5-methyluracil) as its second pyrimidine base (along with cytosine), while RNA uses uracil (lacking the methyl group thymine has) instead of thymine. This structural distinction (thymine in DNA vs. uracil in RNA) is one of the defining chemical differences between the two nucleic acids, alongside the sugar (deoxyribose vs. ribose) and typical strandedness (double vs. single).