(d) : The enzyme restriction endonuclease EcoRI is found in the colon bacteria E. coli. So, here 'co' stands for coli. According to nomenclature of restriction enzyme, the first letter used for the enzyme is the first letter of the genus name (in italics) of the bacterium, then comes the first two letters of its species (also in italics), next is the strain of the organism. At last is a Roman numeral signifying the order of discovery. Here, the enzyme EcoRI was isolated from the bacterium Escherichia coli (co), strain RY13(R) and it was first endonuclease (I) isolated from E.coli.