A 30 year old unmarried woman of average socio-economic background believes that her boss is in secretly love with her. She rings him up at odd hours and writes love letter to him despite his serious warning not to do so. She holds this belief despite contradiction from family members & his denial. However she is able to manage her daily activities as before. She is most likely to be suffering from -
Depression
Shizophrenia
Delusional disorder
No psychatric ailment