Can I place a subpanel next to a water pipe?
The code requires a 30" wide 36" deep working space in front of the panel which can be measured from either edge of the panel. The vertical pipe would likely not be an issue, but for location A the horizontal stuff on the floor would be.

You can mount directly to the concrete if you have the tools, but that creates difficulty securing the cables where they leave the panel. 

You may need to consider how you local Authority Having Jurisdiction interprets the NEC section prohibiting using NM cable were subject to physical damage. Some jurisdictions have adopted specific local codes that specify everything less than 7 or 8 feet is considered subject to damage. The areas you show your subpanel options are lower than your service panel, and may cross the line in your AHJ's opinion.