Explanation:
The correct answer is option 'D', Legend.

Legend:
A legend is a chart element that is utilized to identify the data series by its color patterns or other visual representations. It provides a key to understanding the different elements or categories represented in the chart. The legend typically appears on the right or left side of the chart.

Identification of data series:
When creating a chart, different data series are often represented by different colors or patterns to make it easier for the viewer to distinguish between them. The legend helps in identifying these data series by associating each color or pattern with a specific category or data set.

Example:
For example, let's say we have a bar chart that represents the sales performance of different products for the year. Each product is represented by a different color bar. The legend will display the name of each product along with its corresponding color, allowing the viewer to easily identify which bar represents which product.

Importance:
The legend is an important chart element because it helps in interpreting the chart accurately. Without the legend, it would be difficult for the viewer to understand which data series corresponds to which category or data set. It provides a visual reference that aids in comprehending the information presented in the chart effectively.

Other chart elements:
While the other options mentioned in the question (chart title, marker, data labels) are important chart elements, they do not specifically serve the purpose of identifying data series by their color patterns.

- The chart title provides an overall description or title for the chart, but it does not directly associate colors or patterns with data series.
- Markers are used to represent individual data points on a chart, but they do not necessarily convey information about different data series.
- Data labels are used to display specific values or labels on a chart, but they are not primarily used for identifying data series by their color patterns.

Therefore, the correct answer is option 'D', Legend, as it is the chart element specifically designed to identify data series by their color patterns.