**Core Concept**
The question is testing the understanding of the DNA replication process, specifically the repair of gaps on the lagging strand. During DNA replication, the lagging strand is synthesized discontinuously in short segments called Okazaki fragments, which are then joined together by DNA ligase.

**Why the Correct Answer is Right**
DNA ligase is an enzyme that seals the gaps between Okazaki fragments on the lagging strand by forming a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the adjacent nucleotide. This process is essential for the completion of DNA replication and the repair of damaged DNA. DNA ligase is a crucial enzyme in the DNA repair pathway, and its activity is necessary for maintaining the integrity of the genome.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because DNA polymerase is responsible for adding nucleotides to the growing DNA strand, but it is not the enzyme that seals 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 other enzymes to access the template strands. It is not involved in sealing gaps between Okazaki fragments.
* **Option C:** This option is incorrect because topoisomerase is an enzyme that helps to manage the supercoiling of DNA, but it is not responsible for sealing gaps between Okazaki fragments.

**Clinical Pearl / High-Yield Fact**
DNA ligase is a crucial enzyme in the DNA repair pathway, and its activity is necessary for maintaining the integrity of the genome. Deficiencies in DNA ligase can lead to genetic disorders such as xeroderma pigmentosum.

**Correct Answer: C. DNA ligase. DNA ligase seals the gaps between Okazaki fragments on the lagging strand by forming a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the adjacent nucleotide.**