The 9 kg block is then released and accelerates to the right, toward the 5 kg block. The surface is rough and the coefficient of friction between each block and the surface is 0.4 . The two blocks collide, stick together, and move to the right. Remember that the spring is not attached to the 9 kg block. Find the speed of the 9 kg block just before it collides with the 5 kg block. Answer in units of m/s

Complete Question

The complete question is shown on the first and second uploaded image

The velocity is    

Explanation:

The kinetic energy of the 9 kg can be determined by these expression

        Kinetic energy of 9 kg  block = initial energy stored - energy lost as a result of friction

  Now to obtain the initial energy stored

               Let U denote the initial energy stored and

Where x  is the length the spring is displaced

k is the force constant of the string

   Now referring to the formula above

i.e          Kinetic energy of 9 kg  block = initial energy stored - energy lost as a result of friction

and we are told that coefficient of friction  = 0.4 and the mass is 9 kg ,the acceleration due to gravity  this displacement length of spring = 0.6

  Therefore  

the independent variable, or the one that is intentionally manipulated is the speed.

there are no correct statementd on the list of choices you provided.