IMO C.
Let me try to explain my approach for this one.
Addition of 2 numbers will be even when both of them are either even or odd.
Now lets check option 1.
3a + 4b is even. 3a can be either even or odd depending value of a. 4b will be even irrespective b being even or odd. Hence, 3a should be even if the sum has to be even. But, we cannot confirm whether b is odd or even with this statement.
Now lets check option 2.
3a + 5b is even. 3a can be either even or odd depending value of a. 5b can be either even or odd depending value of a. Hence, a and b can be either even or odd. This also not sufficient to conclude whether b is even or off.
Combing these 2 statements.
Both 3a + 4b and 3a + 5b are even.
Then as per first statement 3a should be even.
Then 4b and 5b should be even if the sum has to be even in both the cases.
Then b should be even. Hence, C is the answer.
Please let us know the OA and explanation if I am wrong or ambiguous.
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