The correct combination is (3) A and C. Statements A and C are true, whereas B and D are false.​

A. A. tumefaciens causes crown gall disease and A. rhizogenes causes hairy root disease – TRUE

• Agrobacterium tumefaciens carrying a Ti plasmid induces crown gall tumors on dicot plants, while Agrobacterium rhizogenes with an Ri plasmid induces hairy root disease.​

• This distinction is textbook information and is correct.

B. Region A in Ti plasmid is responsible for replication – FALSE (as stated)

• The Ti plasmid contains an origin of replication (oriV) and rep genes that mediate plasmid replication and maintenance, but these are not traditionally called “Region A” in standard Ti‑plasmid maps.Many teaching diagrams label “Region A” as the T‑DNA region, not the replication origin, so the statement that “Region A is responsible for replication” conflicts with the usual map where replication is a separate ori/rep region.​

• Therefore B is not accepted as correct in this question.

C. Region D in Ti plasmid is responsible for virulence – TRUE

• Ti plasmid diagrams commonly mark one large cluster as the vir region (virulence region), which houses vir genes (virA, virB, virC, virD, virE, virG etc.) essential for T‑DNA processing and transfer.In many exam figures this vir region is labeled “Region D,” making C a correct statement.

D. Oncogenic (onc) region in T‑DNA is responsible for unusual amino acid synthesis – FALSE

• The oncogenic region of T‑DNA carries genes (iaaM, iaaH, ipt etc.) for auxin and cytokinin biosynthesis that cause uncontrolled cell division leading to tumors.​

• Genes for opine (unusual amino acid/sugar derivatives) synthesis, such as ocs or nos, are a separate part of T‑DNA often called the opine synthase region, not the onc region.​

• Thus attributing “unusual amino acid synthesis” to the oncogenic region is incorrect.

Option-wise evaluation

• (1) A and B – Incorrect; B is wrong.

• (2) C and D – Incorrect; D is wrong.

• (3) A and C – Correct; both statements accurately reflect Agrobacterium diseases and the virulence (vir) region of the Ti plasmid.

• (4) B and D – Incorrect; both statements are false.

SEO‑oriented introduction (for article use)

In Agrobacterium‑mediated plant transformation, A. tumefaciens (crown gall) and A. rhizogenes (hairy root) are distinguished by their Ti and Ri plasmids, which carry T‑DNA onc genes for plant hormone overproduction and a separate virulence region (often labeled as region D) with vir genes required for T‑DNA transfer. Opine synthesis genes lie in a distinct opine synthase region, not in the oncogenic region, and replication is controlled by ori/rep sequences, so only statements A and C together describe the Ti plasmid correctly.​