What was the name of the Apollo 13 service module oxygen tank that exploded?

The Apollo 13 service module oxygen tank that exploded was oxygen tank 2.

On April 13, 1970, oxygen tank 2 in Apollo 13’s service module failed during the crew’s journey toward the Moon. The resulting explosion damaged the service module and caused a major loss of oxygen and electrical power. Commander Jim Lovell, command module pilot Jack Swigert, and lunar module pilot Fred Haise were forced to use the lunar module Aquarius as a lifeboat.

The tank had been damaged during an earlier ground operation, and a mismatch involving its electrical heating system contributed to the later failure. The explosion itself was not caused by a meteorite or a collision with the Moon.

Mission controllers developed procedures to conserve power, remove carbon dioxide, and guide the spacecraft around the Moon for a safe return. The crew splashed down in the Pacific Ocean on April 17, 1970.