The target player chooses which creatures he or she will sacrifice as Dead Drop resolves. If the player controls only one creature at that time, he or she will sacrifice it.

Dead Drop doesn’t target any creature. The target player could sacrifice a creature with hexproof, for example.

The rules for delve have changed slightly since it was last in an expansion. Previously, delve reduced the cost to cast a spell. Under the current rules, you exile cards from your graveyard at the same time you pay the spell’s cost. Exiling a card this way is simply another way to pay that cost.

Delve doesn’t change a spell’s mana cost or converted mana cost. For example, Dead Drop’s converted mana cost is 10 even if you exiled three cards to cast it.

You can’t exile cards to pay for the colored mana requirements of a spell with delve.

You can’t exile more cards than the generic mana requirement of a spell with delve. For example, you can’t exile more than nine cards from your graveyard to cast Dead Drop.

Because delve isn’t an alternative cost, it can be used in conjunction with alternative costs.