**Core Concept**
The process being tested is the sealing of gaps between DNA segments on the lagging strand, which occurs during DNA replication. This involves the action of enzymes that repair breaks in the DNA backbone.

**Why the Correct Answer is Right**
During DNA replication, the lagging strand is synthesized discontinuously in short segments called Okazaki fragments, each 1000-2000 base pairs long. The gaps between these fragments are filled in by DNA polymerase and then sealed by DNA ligase, an enzyme that forms a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of another. This seals the gaps and allows the DNA to be replicated accurately.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DNA polymerase is responsible for adding nucleotides to the growing strand, but it does not seal the gaps between Okazaki fragments.
**Option B:** This option is incorrect because DNA helicase is an enzyme that unwinds the double helix structure of DNA, allowing the replication machinery to access the template strand.
**Option C:** This option is incorrect because topoisomerase is an enzyme that relaxes supercoils in the DNA by cutting and rejoining the phosphate backbone, but it is not involved in sealing gaps between Okazaki fragments.

**Clinical Pearl / High-Yield Fact**
DNA ligase is essential for sealing the gaps between Okazaki fragments on the lagging strand, and its activity is critical for maintaining the integrity of the genome. Defects in DNA ligase can lead to genetic disorders, such as severe combined immunodeficiency (SCID).

**Correct Answer:** D. DNA ligase. DNA ligase seals the gaps between Okazaki fragments on the lagging strand.