The correct option is
A Seriousness
A synonym is a word or a phrase that is similar in meaning to another word/phrase.
Option D: 'Earnestness' means 'sincere and intense conviction', 'serious in intention or purpose'.
'Seriousness' means 'being in earnest', 'sincere'.
You can see that 'seriousness' means the same as 'earnestness'.
Both mean 'sincere and honest'. Hence option D is correct.
Option A: 'Quickness' means 'the quality of doing something fast in a short time'.
Option B: 'Alertness' means 'the quality of being aware about your surroundings'.
Option C: 'Tenderness' means 'the quality of being gentle and kind'.
These words do not mean the same as 'earnestness'.
Hence, options A, B and C are wrong.