In divide and conquer for maximum sum subarray, the array is divided into two halves.
The maximum sum subarray can occur in three possible ways.
Case $1$: It lies completely in the left half.
Case $2$: It lies completely in the right half.
Case $3$: It crosses the middle.
If it crosses the middle, then it must include:
- a suffix of the left half
- a prefix of the right half
So, the crossing sum is calculated by taking the best suffix sum from the left side and the best prefix sum from the right side.
Therefore, the third possible case is:
It crosses the middle, using a suffix of the left half and a prefix of the right half.
Answer: C