Answer :
During the transcription process, nucleotides are complementary to each other when there's a purine (double-ringed nucleotides, such as Adenine and Guanine) and a pyrimidine (single-ringed nucleotides, such as Cytosine, Thymine, and Uracil). This allows the genetic material only to pairs Adenine with Thymine, due to these nucleotides having only two hydrogen bonds that can share one with the other. Whilst Guanine can pair only with Cytosine due to these nucleotides having three hydrogen bonds that they can share with the complementary base.