**Answer:**

The correct answer is option 'A': DNA polymerase.

DNA polymerase is an enzyme that plays a crucial role in DNA replication. It adds complementary nucleotides to the growing DNA strand during the DNA synthesis process.

**DNA Replication:**

DNA replication is the process by which a cell makes an identical copy of its DNA. It is a vital process for cell division and the transmission of genetic information from one generation to the next. DNA replication occurs in the nucleus of eukaryotic cells.

**Role of DNA Polymerase:**

DNA polymerase is the primary enzyme involved in DNA replication. It catalyzes the addition of nucleotides to the growing DNA strand, ensuring that the new DNA molecule is a faithful copy of the original.

During DNA replication, the double-stranded DNA molecule unwinds, and the two strands separate. Each separated strand serves as a template for the synthesis of a new complementary strand. DNA polymerase binds to the template strand and adds complementary nucleotides to the growing DNA chain.

**Complementary Base Pairing:**

In DNA, the nucleotides are composed of four different bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The bases pair up in a specific manner: A always pairs with T, and C always pairs with G. This is known as complementary base pairing.

When DNA polymerase adds nucleotides to the growing DNA strand, it ensures that the complementary base is added to the template strand. For example, if the template strand has an A, DNA polymerase will add a T to the new strand. This process ensures the fidelity of DNA replication.

**Conclusion:**

In summary, enzymes called DNA polymerase add complementary nucleotides to the growing DNA strand during DNA replication. They ensure that the new DNA molecule is an accurate copy of the original by following the rules of complementary base pairing.