Identify the odd number: ​​

Logic: 1st digit +  2nd digit = 9

Now, we check each options.

Option (a): 28 (Not Follow)​

2 + 8 $$\neq$$ 9​

Option (b): 45 (Follow)

5 + 4 = 9

Option (c): 63 (Follow)​

6 + 3 = 9

Option (d): 36 (Follow)​

3 + 6 = 9 

Thus, correct option is (a).