Type of nitrogenous base with a double ring found in DNA and RNA:

a. Ribose

b. purine

c. pyrimidine

d. cytosine

The correct answer will be option-B (purine).

Explanation:

Nucleic acid structure is made up of monomer unit called nucleotide. Each nucleotide is composed of a five-carbon sugar, a phosphate group and five types of nitrogenous bases.

The nitrogenous bases are categorized in two forms the purines (adenine and guanine) and the pyrimidines (cytosine, thymine and uracil). The "purines" contain double rings in their structure due to which they appear larger in size than pyrimidines which are made up of single rings.

Thus, Option-B is the correct answer.