The correct option is (3): placement of the passenger gene towards the LEFT border and the marker gene towards the RIGHT border of T-DNA.

Reasoning: T‑DNA transfer starts at the RIGHT border and proceeds toward the LEFT border. Truncations are therefore more likely to occur near the left border; any DNA closer to the left border has a higher chance of being lost. If the selectable marker is near the right border and the passenger gene is near the left border, many events can still carry an intact marker (so they survive selection) but lose part or all of the passenger gene, giving incomplete transfer.

Explanation of each option:

1. Marker near left border, passenger near right border

• The passenger gene lies near the right border, where transfer initiates, so it is more likely to be included even if deletions occur toward the left end.

• This arrangement reduces, rather than increases, the probability of incomplete passenger gene transfer.

2. Marker under constitutive promoter, passenger under tissue‑specific promoter

• This alters expression patterns, not how completely the T‑DNA is transferred.

• It may affect where/when the passenger gene is expressed, but it does not preferentially generate truncated passenger genes.

3. Passenger near left border, marker near right border – CORRECT

• Transfer initiates at the right border and moves toward the left; deletions or premature termination often remove sequences near the left border.

• Because selection relies on the marker near the right border, many selected plants can still lack part of the more distant passenger gene, giving incomplete transfer.

4. Both genes under constitutive promoters

• Again, this affects expression strength and pattern, not the physical likelihood that one gene is truncated during T‑DNA transfer.

• It does not specifically predispose the passenger gene to incomplete transfer.