Girdling experiment is not successful in monocots due to
Girdling/ ringing experiment:
The experiments in which the continuity of vascular tissues is disturbed by their removal are known as girdling experiments.
It is not successful in monocots plants because they have scattered vascular bundles i.e., that lack organisation. This is called the atactostelic condition. They have a random arrangement of xylem and phloem while dicots have a star-shaped arrangement.
Hence, the correct answer is 'Vascular bundles are nor arranged in a ring'.