The correct option is D imaginary
'Fictitious' means not real or true. So, the meaning of the word is option D 'Imaginary' which also means not real. Option A 'unreliable' means untrustworthy. Option B 'unbelievable' means difficult to believe as it is unlikely. Option C 'insightful' means showing a deep understanding of a complicated problem. Thus, options A, B and C are incorrect as meaning of 'fictitious' and the correct option is option D 'Imaginary'.