Which symbol does zeus send to odysseus as an omen?

Omens are or foreshadows of future events which as any symbol is up to conventions and particular interpretations.

When Zeus sends down a Thunderbolt across a clear sky and Odysseus reads that as the signal he was asking for, to know he was meant to come home, which helps him regain his courage, the omen is seen by Eurycleia who reads it as the defeat of the greeedy suitors.

Explanation:

b. our party quickly made its way into the cave / but we failed to find our host [the cyclops] himself inside.