An open equation is an equation that contain one or more unknow variables.
And that/those variable(s) can be found by solving the equation that make the equation true.
A variable is an English alphabet.
For example, a,b, x, y,z etc.
A variable represents a unknow value for a particular quantity.
In the given options 5x + 7 = 12 , 12 + (-8) = 4 , 25 = 5 (5) , 8 + 0 =8 .
In first option, we have unknow variable x is there.
But in all other options we don't value any variable.
Therefore, all other options can't be open equation.
So, final answer is 5x + 7 = 12 is the only open equation in given options.