Q.The complicated but efficient method of measuring trend of time series is ______.

The method of least squares fits the trend line $Y = a + bX$ by minimising $\sum (Y - \hat{Y})^2$; it is more complicated to compute but is the most efficient and objective trend-measurement method. Hence option (C).

Reviewing the methods of measuring trend in a time series:

• Graphical (freehand) method — quick and simple but subjective.
• Method of moving averages — smooths data but cannot forecast beyond the series and loses end terms. …