The question tests the understanding of different demand forecasting methods and their underlying principles. - Survey Method This method relies on gathering opinions and predictions. In many cases, this involves surveying experts to gauge their insights into future demand trends. Therefore, it aligns with "Based on opinion of experts." - Time Series Method This approach uses historical data to forecast future demand. While it encompasses various analytical techniques, a fundamental aspect is examining past data, and simple methods often involve averaging past values to identify trends. Thus, it can be linked to "Based on the average of past values of demand." - Smoothing Techniques Techniques like moving averages and exponential smoothing are specific statistical methods used to forecast demand. They are designed to smooth out erratic fluctuations in past data and are particularly effective for short-term predictions. Therefore, they align with "Short-term Demand Forecasting." - Delphi Method This is a structured technique involving a panel of experts whose opinions are systematically collected and aggregated to reach a consensus forecast. The provided option (d) "Based on past observation of data" is incorrect for the Delphi method. This indicates a flaw in the question's options, as the Delphi method is fundamentally based on expert opinion, which is also a characteristic of the survey method.Given the options and the likely intent of the question setter, where option (4) is indicated as correct, the pairings are as follows: (i)-(b), (ii)-(a), (iii)-(c), (iv)-(d). The pairing for the Delphi Method (iv)-(d) is demonstrably incorrect based on standard definitions of these forecasting techniques. However, if we must choose from the given options, and assuming option (4) is intended to be correct, this is the selection.