The correct equation that represents the conservation of mechanical energy for the system of a marble attached to a compressed horizontal spring is:
\(\frac{1}{2} mv^2 = \frac{1}{2} k x^2\)
This can also be written as:
\(12mv^2 = 12kx^2\)
This equation indicates that the kinetic energy (\(\frac{1}{2} mv^2\)) of the marble when the spring is released is equal to the potential energy stored in the spring when it is compressed (\(\frac{1}{2} k x^2\)).
So the second response you mentioned is the appropriate choice.