Where does the letter ‘R’ in EcoRI come from?

The correct answer is Option (4) → From the name of strain.

Restriction enzymes are named from the bacterium they come from. The first letter is from the genus, the next two letters are from the species, the next letter is from the strain, and the Roman number gives the order in which the enzymes were isolated from that strain. EcoRI comes from Escherichia coli RY 13: E from Escherichia, co from coli, R from the strain RY 13, and I because it was the first enzyme isolated from that strain.

NCERT Extract:
Restriction Enzymes – “… The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated, e.g., EcoRI comes from Escherichia coli RY 13. In EcoRI, the letter ‘R’ is derived from the name of strain. Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria.”