Assume we have 8 vertices from 1 to 8
8 is connected to 7 6 5 4 3 2 1 --> degree = 7
7 is connected to 8 6 5 4 3 2 --> degree = 6
6 is connected to 8 7 5 4 3 --> degree = 5
5 is connected to 8 7 6 4 --> degree = 4
4 is connected to 8 7 6 5 --> degree = 4
3 is connected to 8 7 6 --> degree = 3
2 is connected to 8 7 --> degree = 2
1 is connected to 8 --> degree = 1
In question they have given 1 + 2 + 3 + 4 + 5 + 6 + 7 + x
Here x = 4
Hence D is the answer