there are 25 grandparents there were in the survey. 4 prefered digital copies, 21 prefered hard copies.
4/25 prefered digital copies
21/25 prefered hard copies
21 is more than 4.
The only one of the possible answers that imply that more people prefered hard copies was the third option. Therefore, "More grandparents prefer using a hard copy than a digital copy." is the most likely to be true.