Only the top card of my opponent's security card is face up. If this card's [When Digivolving] effect activates, what security card is flipped to face up?

An opponent's Digimon attacks a player by an effect at the end of their turn. What happens if this card's {Security} [End of Opponent's Turn] effect then plays this card, my security stack is reduced to 0 cards, and the attack against the player is successful?