The correct answer is B. Permanent wilting point.
The permanent wilting point is the point at which plants can no longer extract water from the soil and will wilt permanently. This occurs when the soil water potential is below -15 bars. The permanent wilting point is important because it is the lower limit of available water for plants.
The maximum saturated point is the point at which the soil is completely saturated with water. This occurs when the soil water potential is zero. The maximum saturated point is not important for plants because plants cannot extract water from saturated soil.
The ultimate utilization point is the point at which plants have extracted all of the available water from the soil. This occurs when the soil water potential is -15 bars. The ultimate utilization point is not important for plants because plants cannot extract water from soil with a water potential below -15 bars.
None of these is the correct answer because it does not describe the state of the soil when plants fail to extract sufficient water for their requirements.