Question

what does the word chronic mean?
happening in an official or formal way
happening in a way that is difficult to believe
happening in a vexing or disturbing way
happening frequently or for a long duration

Explanation:

The term "chronic" is defined as something that occurs over a long period of time or repeatedly, which matches the last option. The other options describe unrelated concepts: official/formal relates to "official" terms, hard to believe relates to "incredible", and vexing/disturbing relates to "troublesome".

Answer:

happening frequently or for a long duration