Ran into this situation, and we were not sure how to resolve it.
Tap: When target creature other than Adarkar Valkyrie dies this turn, return that card to the battlefield under your control.
Player B casts Necromantic Selection, which reads
Destroy all creatures, then return a creature card put into a graveyard this way to the battlefield under your control. It's a black Zombie in addition to its other colors and types. Exile Necromantic Selection.
Player A responds to Player B casting Necromantic Selection by activating Adakar Valkyrie's ability, targeting the Sun Titan. Is it possible for Player B to get the Sun Titan instead of Player A?