Final answer:
Adenine forms a complementary base pair with thymine in DNA, stabilized by two hydrogen bonds, which helps to maintain the DNA's double helix structure.
Step-by-step explanation:
The base that forms a complementary base pair with thymine is adenine. In the structure of DNA, the nitrogenous bases pair between the two strands forming the double helix. Each nucleotide has a deoxyribose sugar, a phosphate group, and a nitrogenous base. Thymine, a single-ringed pyrimidine, pairs specifically with adenine, a double-ringed purine, via hydrogen bonds. Adenine and thymine share two hydrogen bonds to maintain the uniform shape of the DNA double helix.