The sentence "Most were first-time users of a tablet and a digital app" would fit best in blank 4 because it provides information about the specific group of people who were involved in the pilot study for the app. The preceding sentence mentions that the app was piloted in early 2021 with over 400 women from rural areas, and the added sentence provides further detail about the specific experiences that these users had with technology. This information helps to clarify that the pilot study for the app involved a group of people who may be unfamiliar with digital devices and services, and it also helps to emphasize the potential impact that the app may have had on these users. Placing the sentence in blank 4 is logical because it provides relevant and specific information about the pilot study for the app, which is the topic being discussed in that part of the paragraph. By contrast, placing the sentence in any of the other blanks would be less logical because it would be introducing information that is not directly related to the main topic being discussed in those parts of the paragraph. Hence, Option D is the correct choice.