Police bring a 30- year- old homeless man to the psychiatric emergency department after he called 911 b/c he thought people were controlling his mind through "microwaves". He has no previous psychiatric or medical hx, and his urine toxicology screen is negative for illicit substances. On interview, he appears very guarded, with limited cooperation, b/c he feels that he does not belong on a psychiatric unit. "I am not the problem", he says. "These microwaves are all around us, even if you cannot feel them." He denies any mood symptoms. Until a few years ago he was working full time, but he refuses to divulge his occupation "b/c I do not trust anyone". On examination, his speech is fluent with normal volume and rate, but his thought process is occasionally disorganized. His affect is blunted, and no psychomotor abnormalities are noted. He denies any hallucinations, but during the interview he occasionally stares behind the interviewer very intently, without responding to any questions. Which term best describes the man's experience of "microwaves"?

A. Delusion.
B. Hallucination.
C. Ideas of reference.
D. Illusion.