(D) Increased speed and accuracy of print
The Gutenberg press could print 250 sheets per hour, enabling rapid, accurate reproduction of texts, making 15th-century printed materials widely available for research.
Source: Print Culture and the Modern World, Section 2.1
The question links 15th-century print culture directly to Gutenberg's press. The key advantage for a researcher is that mechanical printing produced large numbers of accurate copies quickly (20 million copies by 1550), making primary sources accessible. Options A, B, C relate to manuscripts or handwriting, which printing replaced—so they are incorrect. Examiners expect you to connect the historical development of print to its practical research benefit.