Final answer:
Decreasing the length of a climate data set by using only recent years is least likely to improve trend detection, as it can lead to misrepresentative conclusions about long-term climate change. Hence, the correct answer is option (a).
Step-by-step explanation:
The technique that is least likely to improve your ability to detect a trend in a climate data set is a) Decreasing the length of the data set by using data just from recent years. This is because a shorter time frame can lead to conclusions that don't reflect long-term trends. Cherry picking a short period of data can be misleading and may show countertrends that do not represent the overall direction of climate change.
Long-term data sets provide a more comprehensive perspective on climate change and help distinguish natural variability from underlying trends. In contrast, focusing on regional averages, analyzing temperature anomalies, and using more accurate instruments can provide a more reliable analysis of climate trends.