Why are Dalits considered "untouchable?"

They are ritually impure and their impurity is contagious.

They are uneducated and not welcome to mix with educated people.

They choose to do the jobs that no one else wants to do.

They carry disease and have low immunity to disease.

Dalits are part of the Indian culture wherein they are considered as the ones who do the dirty work such as scavenging, and removal of human wastes and dead animals. Therefore, they are considered "untouchable" for the main reason that they are ritually impure and the impurity that they carry is contagious. The answer is the first option.