The best choice is (1) lentiviral vector.

Why lentiviral vectors are ideal

• Lentiviruses (e.g., HIV-derived vectors) are engineered to be replication‑defective but retain the ability to integrate the transgene into the host cell genome.

• Once integrated, the transgene is copied during DNA replication and passed to daughter cells, giving stable, long‑term expression over many cell generations, especially when coupled with appropriate promoters and selection.

Therefore, for sustainable expression in successive generations of a cell line, lentiviral vectors are the ideal gene-transfer system.

Why the other options are not ideal

• (2) Adenoviral vector

  • Adenoviruses generally remain episomal (non‑integrated), causing strong but transient expression that is diluted as cells divide.

  • Not optimal for long‑term, heritable expression.

• (3) Plasmid DNA containing the transgene

  • Standard plasmid transfection usually results in non‑integrated episomes unless rare integration events occur.

  • Expression declines over passages as plasmid is lost.

• (4) Only transgenic DNA (naked DNA)

  • Naked DNA has very low efficiency of entry and integration.

  • Any expression is typically short‑lived and not reliably inherited across generations.

Hence, for stable, sustainable expression across successive generations of a cultured cell line, the correct answer is (1) lentiviral vector.