Because the answer for theoretical value in experiment is affected by many factors while the

Your statement is a general truth about theoretical versus experimental values, but it doesn't fully answer the specific question from 3(a)(iii): "Explain why the standard enthalpy change of formation of ethanol cannot be measured directly."

The reason it cannot be measured directly is that the direct reaction of carbon, hydrogen, and oxygen to form ethanol is not selective; it would produce a mixture of many different products, making it impossible to isolate the enthalpy change for ethanol formation alone.