When a system of equations has infinite solutions, you'll end up with a statements that's true no matter what. For example, 6=6. Or when the graphs of the equations are the same exact line.
Then:
3x+4y=12 and
6x+8y=24 divided by 2: 3x+4y=12. That means they are the same exact line, so it has infinite solutions.
The answer would be the first one and the third one.