The root hair plexus is primarily responsible for detecting various stimuli within the soil, which helps the plant in its growth and survival. Among the options provided, the correct answer would be b) pain.
To explain this further, the root hair plexus contains specialized nerve endings that help the plant sense harmful conditions that might affect its roots. This detection is akin to the sensation of pain in animals, as it alerts the plant to potential threats in its environment, such as hostile soil conditions or injury. This response enables the plant to respond accordingly, promoting its overall health and resilience against adverse situations.
While the root hair plexus can also respond to other stimuli like pressure (c) and touch (e), the primary role is more closely aligned with the sensation of pain (b), helping the plant navigate its underground environment.