Spell casting technicalities (Hexproof and targeting)

[TreeTopSky]

Scenario:
I have a creach with: "Discard a card, this creach gains hexproof until eot."
Opponent casts murder (Targeting my creach), as a response i activate my creach's ability giving him hexproof.

Question:
Will the murder fizzle, or not? Im a bit confused after learning that targets are chosen before the costs are paid.

-Tree

[Ekann1]

Yes, the murder will fizzle because your {Prognostic Sphinx} is an illegal target when the spell resolves.

[TreeTopSky]

Yes, the murder will fizzle because your {Prognostic Sphinx} is an illegal target when the spell resolves.

Ha that card tho, knew exactly what i was talking about.

[Ekann1]

The ability is pretty unique and I know the standard cards very well

[Avodroc13]

Not part of the original question, but what's your fav standard card? :3

[Ekann1]

Hmm. Gotta go with Brimaz, Kitty King. I play mono white. A lot.