**Core Concept**
Restriction endonucleases are enzymes used in genetic engineering to cleave DNA at specific recognition sites, allowing researchers to manipulate and modify the genetic material. These enzymes are essential tools in molecular biology, enabling the precise cutting and joining of DNA fragments.

**Why the Correct Answer is Right**
The most commonly used restriction endonuclease is EcoRI, which recognizes the palindromic sequence GAATTC and cleaves the phosphodiester backbone of the DNA at this site. This recognition and cleavage mechanism is mediated by the enzyme's active site, which specifically binds to the target DNA sequence. The enzyme's ability to recognize and cleave this specific sequence makes it a valuable tool in genetic engineering.

**Why Each Wrong Option is Incorrect**
**Option A:** Other restriction endonucleases, such as BamHI and HindIII, are also commonly used in genetic engineering, but EcoRI is the most widely used.
**Option B:** While DNA ligases are essential enzymes in genetic engineering, they are not restriction endonucleases and do not cleave DNA at specific recognition sites.
**Option C:** Restriction endonucleases are not the same as DNA polymerases, which are enzymes involved in DNA replication and repair.

**Clinical Pearl / High-Yield Fact**
Remember that restriction endonucleases are highly specific and can be used to cleave DNA at specific recognition sites, allowing researchers to precisely manipulate the genetic material.

**Correct Answer: A. EcoRI.**