• Thermal Efficiency of Convective Food Drying System:
• Energy Balance Analysis:
• Total energy input supplied to the drying system ($E_{\text{in}}$) = $40\text{ kW}$.
• Energy lost through system walls, exhaust, and uninsulated piping ($E_{\text{lost}}$) = $10\text{ kW}$.
• Net useful energy utilized for moisture evaporation and sensible product heating ($E_{\text{useful}}$):

$$E_{\text{useful}} = E_{\text{in}} - E_{\text{lost}} = 40\text{ kW} - 10\text{ kW} = 30\text{ kW}$$

• System Efficiency ($\eta$):

$$\eta = \frac{E_{\text{useful}}}{E_{\text{in}}} \times 100 = \frac{30\text{ kW}}{40\text{ kW}} \times 100 = 75\%$$

• Conclusion: System efficiency is 75% (Option 3).

Correct option: (3) 75%