Final answer:
Dorothy should include gridlines in her column chart to help guide the eye and improve the chart's readability, as they serve as a reference point making it easier to read values.
Step-by-step explanation:
Dorothy designed a column chart and would like to add an element that will help to lead the eye and improve the readability of the chart. To enhance her chart, Dorothy should include gridlines, which serve as a reference point for the eyes, making it easier to read values across the chart. Data labels offer a direct reading of values, but they do not guide the eye along the chart as effectively as gridlines. While a trendline is important in line graphs to show how data points have changed over time, in a column chart, especially if it's used to present discrete data points, a trendline may not be appropriate. Annotations could provide additional information, but they do not guide the eyes across data points like gridlines do.