A

Step 1: Understanding the System
In this scenario, we have a block sliding down a wedge on a frictionless surface. The entire system (block + wedge) is isolated horizontally, meaning there are no external horizontal forces acting on it.

Step 2: Conservation of Momentum
Since there are no external forces acting horizontally, the horizontal momentum of the system (block + wedge) will be conserved throughout the motion. Therefore, the magnitude of momentum of the block + wedge system in the horizontal direction is conserved, which corresponds to the option [A].

Step 3: Mechanical Energy of the Block
The mechanical energy of the block is not conserved because as the block slides down, gravitational potential energy is converted into kinetic energy. However, some of this kinetic energy will increase the wedge's kinetic energy as well possibly, due to the block moving downwards. Thus, option [B] is correct for mechanical energy, but not for the momentum situation.

Step 4: Analyzing Momentum
The momentum of the entire block-wedge system remains constant over time (horizontally), thus its magnitude does not increase (contrary to option [C]). Option [D] regarding angular momentum of the block about point O needs to be analyzed: which should actually stay constant as long as the wedge doesn't gain significant rotation.

Therefore, we conclude that:
Magnitude of momentum of block + wedge system in horizontal direction is conserved (Option A).