Step 1: Understanding the Concept:
A chart plots data series. Each series has data points shown by data markers.
The blank asks for the item that adds details about a marker.
Step 2: Working Out the Answer:
A data label is text attached to a data marker.
It gives additional information, such as the value or category, to identify the data point in a series.
So option (D), Data label, fits the blank.
Step 3: Why the Other Options Are Wrong:
Option (A): Source data is the data range from which the chart is drawn. It is not an addition to a marker.
Option (B): Secondary data is data collected earlier by someone else. It has no role in a chart marker.
Option (C): Data series is a group of related data points. It is the group, not the extra information.
Final Answer:
The word that fits the blank is Data label. This is option (D).