The correct answer is C.
The Industrial Revolution suddenly tranformed the traditional manufacturing processes into massive production lines comprised in factories. Factories started to demand high numbers of workers, that mostly came from the countryside as there was a surplus of workers in the agricultural sector.
Factories were mostly located in urban areas so there were large migration flows taking place from rural to urban areas. At first, the cities lacked the facilities and services to house such large amounts of people that were arriving, and most of them had to live hacinated and in really subhuman conditions.