This question is all about doubles. The given bottom cards 5 2 3 4 and the top cards are 1 4 3 2.
The problem is asking to match the top cards to the bottom ones then adding 1 to get the doubles.
So 1 should be corresponding to two. Since 1 + 1 = 2
4 should be corresponding to 5. Since 4 + 1 = 5
3 should be corresponding to 4. Since 3 + 1 = 4
And lastly, 2 should be corresponding to 3. Since 2 + 1 = 3