The salt index of urea is:

Correct option is A

The salt index of urea is approximately 1.61 per unit of nitrogen.
Salt index indicates the potential of a fertilizer to increase the osmotic pressure in the soil solution, which can affect seed germination and root growth.
Urea has a relatively low salt index, making it safer for use near the root zone compared to other nitrogenous fertilizers.
Explanation:
(a) 1.61 per unit of N – Correct answer. Reflects urea’s low salt index, suitable for broadcast and top dressing.
(b) 2.99 per unit of N – Value close to that of ammonium nitrate.
(c) 3.25 per unit of N – Salt index value for ammonium sulfate.
(d) 6.00 per unit of N – Much higher than urea; typical of fertilizers like potassium nitrate.