Final answer:
In the context of DNA replication, 'A' represents Adenine, which is a purine nitrogenous base that pairs specifically with Thymine.
Step-by-step explanation:
Using the picture provided of the process of DNA replication, 'A' represents Adenine. This is based on the figure where adenine (A) is shown as a nitrogenous base that pairs with thymine (T) in DNA replication. Adenine is one of the four nitrogenous bases found in the nucleotides of DNA, the others being thymine (T), guanine (G), and cytosine (C). These bases pair specifically according to Chargaff's rules: adenine with thymine and cytosine with guanine. Adenine and Guanine are purine bases with double-ringed structures, while Cytosine and Thymine are pyrimidine bases with single-ringed structures.