Final answer:
To add a data series to a chart in the Chart Tools contextual tab, you should click on "Add Data", which allows you to include new data into your existing chart. The other options mentioned serve different purposes and are not used to add a data series.
Step-by-step explanation:
To add a data series to a chart from the Chart Tools contextual tab, you should click on "Add Data". This option allows you to insert new data into your existing chart. The process typically involves selecting the chart to which you want to add data, then clicking on the 'Add Data' option, and following the prompts to insert the new data points into the series. Options such as 'Delete Data', 'Insert Data Series', or 'Format Chart' have different functions: 'Delete Data' would remove data, 'Insert Data Series' might not be an actual option depending on the software, and 'Format Chart' is generally used to change the appearance of the chart, not to add new data series.