Elongation or lengthening of rosette plants is obtained by treating it with

The correct option is gibberellin.

Rosette plants are characterized by their short internodes and clustered leaves, which grow close to the ground in a circular arrangement. Under normal growth conditions, these plants exhibit very little stem elongation during their vegetative phase.

Gibberellins are a class of plant hormones (phytohormones) that play a key role in regulating growth and developmental processes, particularly stem elongation, seed germination, flowering, and enzyme induction.

When rosette plants are treated with gibberellin, it induces rapid internodal growth and elongation of the stem. This biological phenomenon is referred to as "bolting" and is typically followed by flowering. The hormone promotes both cell division and cell elongation in the stem, thereby overcoming the natural rosette habit of the plant.