A true breeding line of plants is produced by:

The correct answer is Option (2) → Continuous self pollination for many generations

A true-breeding line is one that shows the same trait stably for many generations. It is produced by continuous self-pollination, because each generation then passes on identical alleles.

(1) Hybridisation — Incorrect. This is a cross between two different parents. It gives hybrids, not a pure line.

(2) Continuous self pollination for many generations — Correct. This gives a line that breeds true.

(3) Emasculation — Incorrect. This is the removal of anthers from a flower bud before they burst. It is used to stop self-pollination when a cross is planned.

(4) Artificial hybridisation — Incorrect. The breeder places chosen pollen on an emasculated flower. This makes hybrids and does not keep a line pure.

NCERT Extract:
Mendel’s Laws of Inheritance – “… Mendel conducted such artificial pollination/cross pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.”