The correct combination is (4) A, B and D. Statements A, B and D are true, while C is not fully correct as written.​

Explanation of each statement

A. Ti and Ri plasmids induce crown gall and hairy root disease, respectively – TRUE

• Ti (tumor-inducing) plasmids of Agrobacterium tumefaciens cause crown gall tumors on dicot plants, while Ri (root-inducing) plasmids of Agrobacterium rhizogenes lead to hairy root disease.​

B. Octopine synthase and nopaline synthase enzymes are encoded by T-DNA – TRUE

• The genes ocs and nos, encoding octopine synthase and nopaline synthase, lie within the T-DNA region of Ti plasmids; once transferred to the plant genome, they direct opine synthesis in tumor cells.​

C. All six vir genes (virA, virB, virC, virD, virE, virG) are absolutely required for virulence – NOT FULLY TRUE

• vir genes are essential as a group, but genetic analyses show that mutations in virA, virB, virD, or virG abolish virulence, whereas virC and virE mutants are typically only partially attenuated rather than completely non‑virulent.​

• Therefore it is not accurate to say all six are “absolutely” required; some are critical, others are important but not strictly essential.

D. Almost perfect 25 bp direct repeat sequences flanking T-DNA in Ti and Ri plasmids are essential for T-DNA transfer – TRUE

• The T-DNA is bordered by 25 bp imperfect direct repeats (left and right borders) that act as cis‑acting sequences required for nicking and processing of the T‑strand; experiments show that the border repeat, especially at the right end, is essential for T-DNA transfer.​

Option-wise evaluation

• (1) A, B and C – Incorrect, because C is not strictly true.

• (2) B, C and D – Incorrect, again due to C.

• (3) A, C and D – Incorrect for the same reason.

• (4) A, B and D – Correct; these three statements accurately describe Ti/Ri plasmid diseases, T‑DNA opine synthase genes, and the essential role of 25 bp border repeats in T‑DNA transfer.​

SEO‑oriented introduction (for article use)

In Agrobacterium-mediated plant transformation, Ti and Ri plasmids are responsible for crown gall and hairy root diseases, respectively, and their T‑DNA carries opine synthase genes such as octopine synthase and nopaline synthase that function inside transformed plant cells. Efficient T-DNA transfer additionally depends on conserved 25 bp border direct repeats at both ends of T‑DNA, whereas not every vir gene is absolutely essential for virulence, making A, B and D the correct combination for this question.​