The organisms connected to the substratum possess radial symmetry in all vertical planes. All the animals belonging to cnidaria (e.g. jellyfish) and echinodermata (e.g. starfish) are radially symmetrical and typically sessile of their grownup form. This condition when the organism is attached to the substratum and locomotion is negligible are said to have sedentary mode of life. So, option d is correct.

Radial symmetry is a type of symmetry found in animals where body parts are arranged around a central axis, much like the spokes of a wheel. This arrangement allows the animal to be divided into two identical halves by any plane passing through the central axis. Radial symmetry is commonly associated with a sedentary mode of life, which means that the organism remains fixed in one place and does not move much.

Radial symmetry is usually associated with a sedentary mode of life due to the following reasons:

1. Sensory Perception: Animals with radial symmetry typically have sensory organs, such as tentacles or rays, arranged around their central axis. This arrangement allows them to detect and capture food particles or sense their environment from all directions. By being sedentary, these organisms can maximize their ability to detect and respond to stimuli from all directions without the need for active movement.

2. Filter Feeding: Many organisms with radial symmetry, such as sea anemones and coral polyps, are filter feeders. They rely on the current of water to bring in food particles, which they capture using their tentacles or specialized structures. By remaining fixed in one place, these organisms can position themselves in areas with optimal water flow and maximize their chances of capturing food efficiently.

3. Attachment: Sedentary organisms with radial symmetry often attach themselves to a substrate, such as rocks or other surfaces, using specialized structures like suckers or adhesive discs. This attachment provides stability and prevents them from being carried away by water currents or other external forces. By remaining sedentary, these organisms can maintain their position and avoid the need for active locomotion.

4. Energy Conservation: Being sedentary allows organisms with radial symmetry to conserve energy. Active movement requires significant energy expenditure, which can be minimized by remaining fixed in one place. By conserving energy, these organisms can allocate resources for other essential functions, such as growth, reproduction, or defense.

In conclusion, radial symmetry is usually associated with a sedentary mode of life because it allows organisms to maximize sensory perception, optimize filter feeding, facilitate attachment, and conserve energy.