What option should be selected to apply a solid fill background in blue?

To achieve a solid fill background in blue using theme colors, selecting the fifth option in the second to last row is the correct choice. This option typically corresponds to a specific shade of blue that is set within the theme of your presentation, ensuring consistency with other design elements.

Using theme colors is advantageous because it leverages a pre-defined palette that maintains cohesion throughout the presentation. Each theme color option is selected based on the design and visual impact it can provide, which is particularly important in professional slides to convey information clearly and attractively.

The other selections may not yield the desired shade of blue or could even represent different colors entirely, which would not fulfill the requirement for a solid blue background. Thus, the chosen option is crucial for ensuring that the visual aspect aligns with your design goals.