The option that best explains why the Red Army won the Russian Civil War is:
Although many groups opposed the Bolsheviks, the White Army was not very unified and this made it harder to have success in the war.
The lack of cohesion and unity among the various factions within the White Army made it difficult for them to present a strong, coordinated front against the Red Army. This disunity, combined with effective Bolshevik strategies, including their ability to leverage propaganda and centralize control, ultimately contributed to the Red Army's victory in the civil war.