The correct answer is A. Adenine.
Guanine is a purine base that pairs with adenine, another purine base. This base pairing is essential for the structure of DNA, as it allows the two strands of DNA to form a double helix.
Thymine and cytosine are pyrimidine bases that pair with each other. Uracil is a pyrimidine base that is found in RNA instead of DNA.
Here is a diagram of the base pairs in DNA:
As you can see, the base pairs are complementary to each other. Adenine always pairs with thymine, and guanine always pairs with cytosine. This complementarity is essential for the stability of DNA.
The base pairs are also responsible for the genetic code. The sequence of the bases in DNA codes for the sequence of amino acids in proteins. This is how DNA is able to store and transmit genetic information.