Opacity is the appropriate control when the objective is to change how intense or subdued the existing colors appear. In Tableau, the Color controls on the Marks card include an Opacity slider. Moving this control toward lower opacity makes marks increasingly transparent; increasing opacity makes the applied colors visually stronger and less transparent. Therefore, C is correct. The other options modify different properties. Border adds an outline around supported mark types and does not control the overall intensity of the fill color. Reversed inverts the ordering of a quantitative color palette—for example, changing a sequential palette so that lower values receive the darker intensity and higher values receive the lighter intensity. It changes which values receive each shade rather than controlling overall opacity. Stepped Color converts a continuous quantitative palette into a specified number of discrete color bands or bins. Tableau's current Color Palettes and Effects documentation explicitly identifies Opacity as the setting used to modify the transparency of marks. It separately defines Reversed as inverting the order of colors and Stepped Color as grouping quantitative values into uniform color bins. These distinctions make Opacity the correct setting for modifying visual color intensity.