What is the missing number in the given diagram?

Logic: The sum of numbers inside three circles in each diagonal should be 15.

Checking the first diagonal (top-right to bottom-left):
Numbers in this diagonal: 7, 3, 5
Sum: 7 + 3 + 5 = 15 
Checking the Middle diagonal (top-right to bottom-left):
Numbers in this diagonal: 5, 4, 6
Sum: 5 + 4 + 6 = 15 
Finding the missing number in the last diagonal (3, 8, ? = 15):
Given numbers: 3, 8, ?
Sum: 3 + 8 + ? = 15
? = 15 - (3 + 8) = 15 - 11 = 4 
Thus, the correct option is (b) 4