What is a key characteristic of serif fonts?

A key characteristic of serif fonts is that they have small lines or decorative strokes regularly added to the end of a larger stroke in a letter or symbol within a particular font. This feature, known as a "serif," enhances the appearance of the text and helps guide the reader's eye along lines of text, making it easier to read in printed format. Serif fonts are often perceived as more formal and traditional compared to sans-serif fonts, which are characterized by their lack of these embellishments.

The other options describe characteristics of different types of fonts. For instance, a font that lacks any decorative lines is a defining trait of sans-serif fonts. Fonts primarily used for digital displays tend to be sans-serif due to their clarity on screens. The mention of a geometric appearance typically refers to certain types of modern or display fonts, which focus on geometric shapes rather than the more organic forms of serif fonts. Thus, the defining quality of serif fonts is indeed the presence of the fine lines or strokes that finish off the main letter forms.