Which god does Odysseus ask for help when he is about to shoot the arrow at Antinous?

We've reached Book 22 of The Odyssey. Odysseus has just proved his incredible strength and skill by successfully firing an arrow through a row of axes. Odysseus is disguised as a humble beggar, so the suitors treat him with mockery and contempt, finding him an object of humor rather than a threat. But after Odysseus meets the challenge of firing an arrow through the axes, his true identity is at last revealed. Now it's time to exact a terrible vengeance upon the suitors.
Antinous is the unofficial leader of the suitors; he's certainly the most aggressive of them. He's openly disrespected Odysseus by paying court to his wife and eating him out of house and home. Even worse than that, he's physically Odysseus in his own palace by throwing a stool at him, thinking he was just a beggar. So it must be hugely satisfying for Odysseus to get some payback when he whispers a brief prayer to Apollo and fires an arrow straight through Antinous's throat. Apollo, among other things, is the god of archers. This explains not just Odysseus's prayer, but also the hapless Antinous's suggestion that they adjourn the archery contest until the following day in order to make a sacrifice to the god.