Which system of equations below has infinitely many solutions

We get an infinite number of solutions when we have equivalent equations, for example equations that a scalar multiples of each other.  That's not going to happen for two equations in slope intercept form unless all the terms are the same.  None of those on this list.

So we're left with the second choice, and clearly the second equation is three times the first, with the same solution set, all the points along the line.

Second choice  y=-3x + 4 and 3y = -9x + 12

practice. practice. practice.