How does Necromantic Selection work with Adarkar Valkyrie?
Player B will have the Sun Titan. Adarkar Valkyrie creates a triggered ability on the creature targeted that will trigger when the creature dies. Necromantic Selection will destroy the creature and return it to the battlefield all during its resolution. Only after that does the triggered ability created by the Adarkar Valkyrie go onto the stack. By this time the Sun Titan is already on the battlefield and cannot be returned to the battlefield so the due to this section of rule 603.6:


  During resolution, these abilities look for the object in the zone that it moved to. If the object is unable to be found in the zone it went to, the part of the ability attempting to do something to the object will fail to do anything. The ability could be unable to find the object [...] because it left the zone before the ability resolved ...