Explanation:

The correct answer is option 'A', which states that the 'R' in EcoRI restriction endonuclease denotes the strain. Let's understand this in detail:

Restriction Endonucleases:
Restriction endonucleases are enzymes that recognize specific DNA sequences and cleave the DNA at or near those sequences. They are commonly used in molecular biology techniques, such as DNA cloning and DNA fingerprinting.

EcoRI Restriction Endonuclease:
EcoRI is one of the well-known restriction endonucleases. It is derived from a strain of Escherichia coli (E. coli) bacteria, specifically Escherichia coli RY13.

Strain RY13:
Escherichia coli RY13 is a specific strain of E. coli bacteria. The strain designation 'RY13' indicates the particular variant or isolate of E. coli used to obtain the EcoRI restriction endonuclease. Different strains of E. coli may have different characteristics or properties.

Sequence of Isolation:
The answer option 'B', which suggests that the 'R' in EcoRI denotes the sequence of isolation of the enzyme, is incorrect. The 'R' in EcoRI does not refer to the sequence of isolation. Instead, it represents the strain of the E. coli bacteria, as mentioned above.

Species and Genus:
The answer options 'C' and 'D', which propose that the 'R' in EcoRI denotes the species or genus, are also incorrect. While EcoRI is derived from E. coli, the 'R' does not represent the species or genus. It specifically refers to the strain of E. coli, which is RY13.

In summary, the correct answer is option 'A' because the 'R' in EcoRI restriction endonuclease denotes the strain of E. coli bacteria, which is RY13.