Final answer:
Option (a), Mercenaries in WWI were not the best soldiers to fight for a cause because they lacked the loyalty and commitment to a specific cause or country, unlike national armies driven by patriotism and ideology.
Step-by-step explanation:
During World War I, mercenaries were not considered the best soldiers to "fight for a cause" mainly because mercenaries lack loyalty and commitment to a specific cause or country (option a). These individuals were often hired soldiers whose motivations could be primarily financial, unlike national armies whose soldiers might share a common patriotism or ideological drive.
In a conflict like World War I, which was marked by enormous casualties, morale and the sheer will to fight for one's country became crucial factors for the effectiveness of the military effort. Apart from loyalty issues, using mercenaries would not align with the concept of a 'total war', which required complete national mobilization and commitment from all sectors of society to support the war effort.