## Core Concept
The question tests understanding of molecular biology techniques, specifically the use of restriction endonucleases and DNA ligases in genetic engineering. Restriction endonucleases cut DNA at specific sequences, and DNA ligases are enzymes that join DNA strands.

## Why the Correct Answer is Right
DNA ligases are enzymes that form a phosphodiester bond between the 5' end of one DNA fragment and the 3' end of another, effectively sealing gaps in DNA. This is crucial in genetic engineering for joining DNA fragments cut by restriction endonucleases. The correct answer, **DNA Ligase**, is the enzyme directly responsible for this action.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and does not provide a viable alternative.
- **Option B:** This option is also blank and does not offer a correct enzyme for joining DNA strands.
- **Option C:** Similarly, this option is blank and incorrect.
- **Option D:** This option is blank as well and does not represent a correct mechanism or enzyme for joining DNA strands.

## Clinical Pearl / High-Yield Fact
A key point to remember is that DNA ligase is essential for creating recombinant DNA molecules by joining fragments cut with restriction enzymes. This technique is foundational in genetic engineering and molecular cloning.

## Correct Answer Line
**Correct Answer: D. DNA Ligase**