Which one is purine base?
A. Uracil
B. Thymine
C. Cytosine
D. Guanine

Hint: Purine is a nitrogenous base which contains fused ring-like structures unlike pyrimidines which have simple rings like structures.

Complete answer:
Nucleobases whose other name is nitrogenous bases are biological compounds which contain nitrogen and help in the formation of nucleosides which in turn form nucleotides. Nucleotides form RNA and DNA.
There are two types of nucleobases
• Purines
• Pyrimidines
Purines are fused ring-like structures. They have a nitrogenous group at 6th carbon in adenine and at 2nd carbon in guanine. Pyrimidines are simple ring structures. Pyrimidines and purines complement each other.
There are two types of purines:
• Adenine is known as 6-amino purine
• Guanine is known as 2-amino-6-oxy purine
There are three types of pyrimidines
• Cytosine is known as 2,4-dioxo pyrimidine
• Uracil is known as 2-oxy-4-aminopyrimidine
• Thymine is known as 2,4-dioxy-5-methyl pyrimidine
Adenine is bonded to thymine and guanine is bonded to cytosine. They both are connected by double or triple bonds. DNA and RNA has adenine and guanine. Cytosine is also found in both DNA and RNA whereas uracil is found in RNA and thymine is found in DNA.
Therefore, from above all the options, option D that is guanine is correct. Guanine is the type of purine present in both DNA and RNA.

The correct option is D.

Note: There are two types of purines that are adenine and guanine and three type of pyrimidines that are cytosine, uracil and thymine. Purines are present in both RNA and DNA whereas under pyrimidine cytosine is present in both but uracil is present in RNA and thymine in DNA.