**Core Concept**
The question is asking about an enzyme that facilitates the joining of two substrates, a fundamental concept in biochemistry. This process is known as a condensation reaction, where two molecules combine to form a new compound, often releasing a small molecule such as water or carbon dioxide in the process.

**Why the Correct Answer is Right**
The correct enzyme that joins two substrates is Ligase. Ligases are a class of enzymes that catalyze the formation of a covalent bond between two molecules, resulting in the release of a small molecule. This process is essential in various biochemical pathways, including DNA replication and repair, where ligases seal gaps in the DNA strand by forming a phosphodiester bond between the 3' end of one nucleotide and the 5' end of another. The enzyme DNA Ligase is a well-known example of a ligase that joins two nucleotides together.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a correct enzyme that joins two substrates. It is possible that this option might be related to another biochemical process, but it is not the correct answer in this context.
* **Option B:** This option is not a valid enzyme that catalyzes the joining of two substrates. It might be a different type of enzyme or a compound involved in a different biochemical process.
* **Option C:** This option is not a correct enzyme that joins two substrates. It is possible that this option might be related to another biochemical process, but it is not the correct answer in this context.

**Clinical Pearl / High-Yield Fact**
One important clinical application of ligases is in DNA repair mechanisms. DNA Ligase plays a crucial role in repairing damaged DNA by sealing gaps and restoring the integrity of the DNA molecule. This process is essential for maintaining genome stability and preventing mutations that can lead to cancer.

**Correct Answer:** C. Ligase.