Which styles affect only paragraphs within the sidebar?

The correct choice is based on the specificity of selectors in CSS. The selector that affects only paragraphs within the sidebar is constructed as an ID selector followed by a type selector, denoted as "#sidebar p {}".

In this selector, "#sidebar" targets an element with the ID “sidebar.” The subsequent "p" indicates that the styles contained within the curly braces will only apply to paragraph elements that are direct descendants of this sidebar element. This specificity ensures that no other paragraph elements outside this particular sidebar will be affected by these styles.

Using an ID selector is appropriate in this case because IDs should be unique to a page, clearly identifying one specific element. By targeting paragraphs specifically within that unique element, you maintain focus on styling related solely to that context, making it clearer and more maintainable in your CSS code.

The other options involve class selectors or body selectors that do not specifically isolate the paragraphs within the sidebar in the same way, thus not achieving the desired effect.