• Formula for Field Water-Use Efficiency (WUE):

$$\text{Field WUE} = \frac{\text{Crop Yield (kg/ha)}}{\text{Total Water Used (ha-mm)}}$$

• Given Values:
• Grain Yield = $4.5\text{ t/ha} = 4.5 \times 1000 = 4,500\text{ kg/ha}$
• Water Requirement = $45\text{ cm} = 45 \times 10 = 450\text{ mm}$
• Calculation:

$$\text{Field WUE} = \frac{4,500\text{ kg/ha}}{450\text{ mm}} = 10\text{ kg/ha-mm}$$

• Conclusion: The field water-use efficiency of the wheat crop is 10 kg/ha-mm. (Correct answer: Option 2)