The only correct statement is (2), so the answer is option (2) A transgenic plant containing two linked copies of the transgene in heterozygous condition would segregate 3:1 for the transgenic phenotype on self‑pollination.

Option-by-option analysis

(1) “Negative selection markers need to be expressed only under strong constitutive promoter for development of transgenic plants.” – Incorrect

• Negative selection markers (e.g., codA, barnase, cytosine deaminase) kill or severely inhibit cells when expressed and exposed to a pro‑drug or specific condition.

• They are often driven by inducible, tissue-specific, or moderate promoters to avoid lethality during early transformation and regeneration. Strong constitutive expression is not mandatory and can be problematic.

(2) “A transgenic plant containing two linked copies of the transgene in heterozygous condition would segregate in a 3:1 (transgenic: non‑transgenic) ratio on self‑pollination.” – Correct

• If two copies are tightly linked and always inherited together, the plant behaves as if it carries one effective dominant locus (T/t) for the trait.

• Selfing Tt × Tt → 3 T_ : 1 tt = 3:1 segregation of transgenic to non‑transgenic phenotypes.

• So this statement is correct.

(3) “Agrobacterium‑mediated transfer of T-DNA into a host plant does not require host plant proteins.” – Incorrect

• T‑DNA transfer and integration require several plant proteins for attachment, nuclear import, chromatin targeting, and integration; these interact with the Vir proteins and T‑strand complex.

• Mutations in specific Arabidopsis host factors reduce transformation efficiency, proving that host proteins are essential.

(4) “In a binary vector system of Agrobacterium tumefaciens, the oncogenes are located on the helper plasmid.” – Incorrect

• In modern binary vector systems, the large Ti plasmid is disarmed: oncogenes (tumor‑inducing genes) are removed.

• The binary (T‑DNA) plasmid carries the genes of interest between LB/RB; the separate helper plasmid provides only virulence (vir) genes, not oncogenes.

• Thus oncogenes are not on the helper plasmid.

Why option (2) is correct

• Among the four statements, only the one about 3:1 segregation from two linked, heterozygous copies is genetically accurate.

• The statements about negative markers, host protein requirement, and oncogenes in the helper plasmid are all incorrect, so the only correct choice is option (2).