The long and the short of it was we are not sure. The B5N level bombers flew and attacked in formation and the timeline was so compressed that it makes it nigh impossible to state with certainty which Kate's bomb penetrated the Arizona's magazine. Gordon Prange credited a Soryu B5N piloted by Sato Haruo and bombadier Kanai Noburo on basis of the latter's flight diary when the Wake Island defenses shot them down on 23 December 1941. Kanai was one of the IJN's champion bombardiers and surviving IJN veterans testified that both Sato and Noburo were the cream of the the Second Carrier Division's level bomber crews. The diary though contradicts the official record that states Sato's formation hit West Virgnia and Tennessee, not Arizona. The IJN's official commendation went to Hiryu commander Kusumi Tadashi who led a second flight of five B5Ns that attacked Arizona soon after the first flight of Kaga B5Ns. The Pearl Harbor researcher David Aiken on the j-aircraft.com forum noted that one aircraft of this five plane formation hit Arizona, another hit Vestal, and the other three missed. In a later thread, Mike Wenger observed:

There seems to be somewhat of a consensus that it was dropped by a horizontal bombing aircraft commanded by Lt.Cdr. Kusumi Tadashi from Hiryu. However, Kusumi was a pilot, not a bombardier. Kusumi's bombardier was Lt.(jg) Kondo Shojiro.

A bit more complicated is the issue of who ordered the release of the bomb. While there is some bit of disagreement regarding who ordered the drop, it was likely the "expert" bombardier in the #2 aircraft. In the (hopefully likely) event that was the case, the culprit was PO1c Kobayashi Masamatsu (I am not 100% positive regarding his given name translation). I would have to say that this was the likely scenario, based on information I have gleaned, and particularly, based on John DiVirgilio's superb research.

So, take your pick!

Kusumi - plane commander.

Kondo - plane bombardier.

Kobayashi - signalled for the drop.

Kusumi died at Midway according the book Shattered Sword. There is little information on either Kondo or Kobayashi, but it is likely they died at Midway as Hiryu's air group had the highest casualties of the four Japanese carriers in the battle.