Step 1: Understanding restriction endonucleases.
Restriction endonucleases, also known as molecular scissors, are enzymes that cut DNA at specific recognition sequences. They are widely used in genetic engineering for creating recombinant DNA molecules.
Step 2: Explanation of other options.
Option (A): Incorrect. DNA ligase joins DNA fragments but does not cut them.
Option (B): Incorrect. Ligase also functions in joining DNA fragments and is not a molecular scissor.
Option (D): Incorrect. RNA polymerase synthesizes RNA and does not cut DNA.
\[
\therefore \text{The correct answer is: Restriction endonuclease.}
\]