Step 1: Understanding the Concept:
Instead of starting from the formula, let us reason from the two end points of the NIV scale and work inward to find where medium status must sit.
Step 2: Key Formula or Approach:
NIV is built so that a value of 1 means every tested sample is low in that nutrient, and a value of 3 means every sample is high. Low, medium, and high fertility classes are laid out as three bands across this 1 to 3 stretch.
Step 3: Detailed Explanation:
If low status sits close to the 1 end and high status sits close to the 3 end, medium status has to occupy the middle stretch of the scale, centered somewhere around 2.
Checking the four options, 0.5-1.0 lies outside the scale entirely, since NIV never drops below 1.
1.0-1.5 and 1.5-2.0 both sit too close to the low end to represent a true middle band.
1.5-2.5 is centered right around 2, comfortably placed between the low and high extremes, which is exactly what a medium classification band should look like.
Step 4: Final Answer:
The medium nutrient status band is 1.5 to 2.5, so option 4 is correct.
\[ \boxed{1.5-2.5} \]