The Roman army was partly comprised of legions, which were . Specialist, non-citizen soldiers citizen-based infantry medical specialists trained arena fighters

Option C, citizen-based infantry

Explanation:

The legions were composed on Roman citizens. There were total of 28 Legion with 4200 infantry in the republican period and was later expanded to 5200 legion in the imperial period. These legion were prepared against the Manipular Army.

Hence, option C is correct