Answer: B. 3/4 inch
Explanation:
Given: The width of each poster = 4 inches
Then the sum of width of 3 posters =

Let x be total width of the advertisement.
Since the ratio of the sum of the widths of the three pictures to the total width of the advertisement is 4:5.
Then, we have

Now, the sum of lengths of empty spaces between the pictures
=Total width - sum of width of 3 posters

From the given picture, we have

Hence, the length of each of the four empty spaces between the pictures = 3/4 inch.