Which one of the mentioned characters does not represent the true breeding line?

A true breeding line is one that shows the X-linked recessive trait.

The correct answer is Option (2) → A true breeding line is one that shows the X-linked recessive trait. 

True breeding line means when an organism is self-pollinated (or self-crossed), it produces offspring identical to the parent for a given trait, generation after generation.

"A true breeding line is one that shows the stable trait inheritance." Correct (that’s the definition).

"A true breeding line is one that shows the X-linked recessive trait." Incorrect.

• X-linked recessive trait is about mode of inheritance, not about true breeding.
• A true breeding line is not defined by being recessive or dominant, autosomal or sex-linked.

"A true breeding line is one that having undergone continuous self-pollination." Correct (selfing stabilizes the line).

"A true breeding line is one that shows stable expression for several generations." Correct (stable inheritance = true breeding).