Answer: The group that was considered effective against the Union Army, but very dangerous because of the use of guerilla tactics were; The Quantrill gang. The correct answer is B.
Step-by-step explanation:
The Quantrill gang were also known as the bushwhackers, they assembled up to 400 men during the Centralia attacks/massacre. They gained the most notoriety as being guerilla fighters. The man who led the gang was William Quantrill. They were an active gang from 1861 until May 1865. Jesse James and his brother were also in this gang of raiders. When the gang was in Lawrence, Kansas, many members and even the women associated with the gang were arrested for their crimes. After the arrest, the gang attacked the town and killed around 180 of the citizens.
He did lose control of this group of men in 1864 when they decided to split apart and go their separate ways.