**Core Concept**
Genetic engineering is a field of biotechnology that involves the manipulation of an organism's genes using biotechnology. This process involves the use of various tools and techniques to modify an organism's DNA sequence, which can be used to introduce new traits or characteristics into the organism.

**Why the Correct Answer is Right**
The most important tool used in genetic engineering is restriction endonucleases. These enzymes recognize specific DNA sequences and cleave the DNA at those sites, allowing for the manipulation of the DNA sequence. This is a crucial step in the process of genetic engineering, as it enables researchers to isolate specific genes or DNA sequences and introduce them into an organism. Restriction endonucleases are highly specific and can recognize a wide range of DNA sequences, making them a powerful tool in the field of genetic engineering.

**Why Each Wrong Option is Incorrect**
**Option A:** PCR (Polymerase Chain Reaction) is a technique used to amplify specific DNA sequences, but it is not a tool used to manipulate DNA sequences.
**Option B:** DNA ligase is an enzyme that seals gaps in DNA, but it is not a primary tool used in genetic engineering.
**Option C:** Gel electrophoresis is a technique used to separate DNA fragments based on size and charge, but it is not a tool used to manipulate DNA sequences.

**Clinical Pearl / High-Yield Fact**
One important thing to remember is that restriction endonucleases are highly specific and can recognize a wide range of DNA sequences, but they can also be inhibited by certain chemicals or conditions, such as high salt concentrations or the presence of certain metal ions.

**Correct Answer: A. Restriction endonucleases**