The correct option is
D Genius
Option D: A 'genius' is 'a person having an extraordinarily high level of intelligence'.
'Brilliant' means 'extremely intelligent or skilled'.
As you can see, 'brilliant' means the same as 'genius'.
'Genius' completes the sentence in meaning: a brilliant person is called a genius.
Hence, option D is correct.
Option A: 'Generous' means 'someone who is very helpful, giving and kind'.
Option B: 'Genial' means 'friendly and pleasant'.
Option C: 'Cordial' means 'friendly, but formal but polite'.
These words do not mean the same as 'brilliant'.
Hence, they're not suitable options.
So, options A, B and C are wrong.