Morphactins are

The correct option is "synthetic growth regulators".

Step-by-step Explanation:

1. Definition of Morphactins: Morphactins are a group of synthetic growth substances that act as inhibitors or regulators of plant growth and development. They are derived from fluorene-9-carboxylic acid.

2. Mode of Action: Unlike natural plant hormones (such as auxins, gibberellins, cytokinins, abscisic acid, and ethylene), morphactins are synthesized artificially. They primarily inhibit or modify growth by interfering with the polar transport of auxins in plants, leading to a variety of morphogenetic changes such as the loss of geotropism and phototropism.

3. Classification: Because they are synthesized artificially and regulate plant growth by modifying normal development, they are classified broadly as synthetic growth regulators rather than being classified specifically as synthetic auxins or synthetic gibberellins.