What adjustment would Celtic scribes make if words do not reach the margin?

The correct choice reflects the practice of elongating some letters to ensure that the text reaches the desired margin. In calligraphy, especially in the context of Celtic scribes, it is not uncommon to make visual adjustments to fit a line of text into a specific space. By elongating certain letters, scribes can make better use of the available space without compromising the overall aesthetics of the script. This technique allows for a more fluid and visually appealing line of text, maintaining the integrity of the calligraphic style.

Other options would not address the issue in the same way. Reducing letter height or using a smaller font could lead to a loss of legibility and might disrupt the visual harmony of the script. Adding additional spacing might create gaps that are undesirable in a tightly composed piece of calligraphy. Therefore, elongating some letters serves as a practical and artistic solution to the problem of text not reaching the margin.