Step 1: Concept
In a number grid, a rule usually acts along every row or every column. We test the easy rules first, such as addition or subtraction of the outer numbers.
Step 2: Try the middle minus first
Row 1: 13 - 7 = 6, which is the last number.
Row 2: 22 - 4 = 18, which is the last number.
So each row satisfies: middle number = first number + last number.
Step 3: Use it for row 3
For row 3, middle = 17 + 7 = 24. We can also write this as: last number = middle - first, that is 7 = ? - 17, so ? = 24. Both give the same value.
Step 4: Test all options
Only 24 makes 24 - 17 = 7 true. The other options give 23 - 17 = 6, 25 - 17 = 8 and 21 - 17 = 4, none of which equals 7.
Final Answer:
The missing number is 24, option 4.
\[ \boxed{24} \]