A

To solve this problem, we'll analyze the system given in the diagram and the relationships between the various parameters listed in Column-I and Column-II.

1. **Velocity of center of mass**: For two identical masses ($m_1 = m_2 = 2$ kg) moving towards each other, the center of mass velocity ($v_{cm}$) can be calculated. Since they move with equal and opposite momenta, the total momentum is zero and thus, the center of mass remains at rest. Therefore, the center of mass does not have velocity, and it's defined as 0 SI unit.

2. **Velocity of combined mass when compression in spring is maximum**: When the spring is maximally compressed, the kinetic energy of the system is at its minimum (essentially, it has converted to potential energy in the spring). The velocity at this point becomes 0, as the masses come momentarily to rest before being pushed apart. Thus, this would also relate to a velocity of 0 SI unit.

3. **Maximum compression in the spring**: To find the maximum compression, we use the principle of conservation of energy. The kinetic energy before the masses collide (which can be calculated by using the velocities of the masses before impact) is converted to potential energy stored in the spring. Given the values, we can derive the maximum compression based on these factors.

4. **Maximum potential energy stored in the spring**: The potential energy stored in the spring at maximum compression is calculated from the equation $PE = \frac{1}{2} k x^2$, where $k$ is the spring constant and $x$ is the compression. Given our previous findings about the compression and velocities, the potential energy can also be derived.

Based on the relationships analyzed, we can match the following:
(i) Velocity of center of mass - **0 SI unit** that corresponds to [D] (since it is not listed clearly we assume it's part of the values),
(ii) Velocity of combined mass at maximum compression is **0 SI unit**,
(iii) Maximum compression in spring relates closely to the maximum kinetic energy values -typically corresponds to **4 SI unit** due to the energy transformations during compression,
(iv) Maximum potential energy correlates to a maximum impact factor of kinetic energy combined, which directly links to **0.5 SI unit**.

Summarily, the answer concludes to A (Velocity of center of mass to [A]).