The first isolated restriction endonuclease was:

The correct option is EcoRI.

Explanation:
Restriction endonucleases (often called molecular scissors) are enzymes that cut double-stranded DNA at specific nucleotide sequences known as recognition sites. EcoRI was one of the earliest and most famous restriction enzymes to be isolated and characterized from the bacterium Escherichia coli (strain RY13). It plays a fundamental role in recombinant DNA technology by recognizing the specific palindromic sequence 5'-GAATTC-3' and cutting the DNA to produce cohesive or "sticky" ends, which facilitate the joining of different DNA fragments.