The practice of using "interchangeable parts" to manufacture industrial goods became commonplace in the early 19th century mainly because a) southern factories adapted to it very easily. b) slavery became illegal during this time period. c) of the shortage of iron and steel that existed following the civil war. d) it meant that great numbers of unskilled laborers could be hired to make industrial products.

The correct statement is "It meant that great numbers of unskilled laborers could be hired to make industrial products."

The early 19th century (aka 1800's) provided America with interchangeable parts. The good thing about these interchangeable parts was that the creation of them consisted of a routine. This meant that unskilled workers could be trained to do one simple task and that would be their sole responsibility for the work day. They would repeat this task over and over again, which lead them to become more efficient at this task over a short amount of time.

The other answers cannot be correct because:
A) Slavery was still widespread in America until the late 1800's (wasn't outlawed until 1865).
B) Southern states had a very few amount of factories during this time.
C) The Civil War didn't happen until the middle/end of the 1800's (1861-1865).