The correct answer is b. Achilles, himself has no peer in war among Achaean [Greek] captains.
It could not have been Hector precisely because he was not an Achaean captain. Indeed, Hector was a Trojan Prince, son of Priam and Hecuba.
In book 21, before Achilles statement, Patroclus was already killed by Hector and the Trojans, who thought he was Achilles because Patroclus was wearing his armor. Besides, Patroclus was not a captain.
Agamemnon was one of the greatest warriors of the age, almost to the extent of Achilles but he was not as good as him. Prove of this is that when he stole from Achilles one of the spoils of war, a slave called Briseis, Achilles withdrew from battle putting the Greek armies at risk. Eventually, Agamemnon sent ambassadors begging Achilles to return to battle. Achilles only returned when he learnt that Patroclus had been killed.
Achilles, being a demigod and a great warrior, almost invincible, is the one who had no peer in war among the Achaean captains and he knew it.