Final answer:
The largest two legions at the time of the Horus Heresy were the Ultramarines and the Imperial Fists. This piece of lore comes from the fictional universe of Warhammer 40,000, where these two legions were known for their numbers and specific combat expertise.
Step-by-step explanation:
The question asks about the largest two legions at the time of the Horus Heresy, a major event in the fictional universe of the Warhammer 40,000 game franchise, therefore, it is not a historical event but rather a part of science fiction lore.
In regard to the student's question, the largest two legions during the Horus Heresy were a) Ultramarines and Imperial Fists.
These two Space Marine legions had the most substantial numbers. The Ultramarines, led by Roboute Guilliman, were known for their strict adherence to the Codex Astartes and their tactical versatility.
The Imperial Fists, commanded by Rogal Dorn, were renowned for their defensive expertise and their stoic nature in battle.
The Horus Heresy is a significant part of the Warhammer 40,000 background, where legendary Primarchs and their legions of genetically-enhanced warriors fought amongst themselves in a brutal galactic civil war.
It is crucial for the students of the Warhammer 40,000 lore to understand the size and capabilities of the various legions involved in this conflict.