In line 102, the publican tells the rioters, "be on your guard with such an adversary." What adversary is he talking about

The correct answer is Death.

Explanation:

The publican refers to death as a great adversary, one everyone should be afraid of.

He says his opponent "has killed a thousand" and "went his way without a word".

He is an inevitable adversary that everyone should be face carefully, but always being prepared, knowing you can meet him any time, and anywhere.

This is an excerpt from the "Pardoner's tale which is one of the Canterbury Tales written by Michael Young. Three young men depart to encounter Death and kill it. The adversary mentioned is "Death".