• Percent (%) represents parts per hundred ($10^2$), while ppm represents parts per million ($10^6$).
• The mathematical relationship between percentage and parts per million is:
  $$1\% = 10,000\text{ ppm}$$
• For a 4% salt solution:
  $$4\% = 4 \times 10,000\text{ ppm} = 40,000\text{ ppm}$$
• Hence, 4% salt concentration equals 40,000 ppm (Option 1).

Field water balance dynamics:

• Assertion (A) is true: The field water balance equation ($\Delta S = [P + I + U] - [ET + R + D]$) tracks all water inflows (precipitation, irrigation, capillary rise) and outflows (evapotranspiration, runoff, deep percolation), along with changes in the root-zone soil water storage ($\Delta S$).
• Reason (R) is false: The primary driving and governing forcing variables of the hydrologic budget are external hydroclimatic inputs (precipitation, irrigation) and evaporative atmospheric demand (potential evapotranspiration), while stored soil moisture is a state variable/consequence rather than the main independent driver.

Calculation of net irrigation requirement:

• Moisture deficit by weight: $\Delta \theta_m = 10\% = 0.10$
• Soil bulk density: $\rho_b = 1.2\text{ Mg/m}^3 = 1.2\text{ g/cm}^3$
• Root-zone soil depth: $D = 0.4\text{ m} = 40\text{ cm}$
• Depth of irrigation water required ($d$):
  $d = \frac{\Delta \theta_m}{100} \times \rho_b \times D = \frac{10}{100} \times 1.2 \times 40\text{ cm} = 4.8\text{ cm}$.