Final answer:
The smallest Astartes Legion in the Warhammer 40,000 universe was the Salamanders, known for their low numbers due to rigorous recruitment standards and significant casualties in key battles.
Step-by-step explanation:
The question is asking which was the smallest of the Astartes Legions, with the options being: a) Alpha Legion, b) Space Wolves, c) Thousand Sons, d) Salamanders.
The smallest Astartes Legion, according to the lore of the Warhammer 40,000 universe, was the Salamanders. There was a multitude of factors that contributed to the Salamanders' low numbers, including their rigorous recruitment processes and heavy casualties during the Dropsite Massacre on Isstvan V.
It's also important to note that the Space Wolves, Alpha Legion, and Thousand Sons each had their own unique histories, but none were noted for being as numerically limited as the Salamanders.