Between which two ends does DNA ligase form a bond?

Correct answer: Between a 3′-hydroxyl and a 5′-phosphate

The governing concept is phosphodiester-bond formation in nucleic-acid ligation. DNA ligase seals a nick between adjacent DNA fragments by joining the 3′-hydroxyl end of one nucleotide or strand segment to the 5′-phosphate end of the neighbouring segment. This creates the continuous sugar-phosphate backbone, so option B is correct. Amino groups belong to proteins and other molecules, not the characteristic pair of DNA termini used here. “Fat ends” and water molecules are unrelated distractors. The reaction also requires suitable end chemistry and an energy source appropriate to the ligase, but the essential pair asked in the question is 3′-OH and 5′-phosphate.