The correct option is D relevant
Option A: 'Abusive' means extremely offending and insulting.
Option B: 'Revealing' means 'to make interesting information known. It is not suitable for this sentence as 'revealing to' ("his answer were (revealing) to") is grammatically incorrect.
Option C: 'Pestilent' means 'deadly, destructive to life'.
Option D: 'Relevant' means 'closely connected or appropriate to something'. This is the most suitable word which fills in the blank. It completes the meaning of the sentence: "Rajeev failed...because none of his answers were relevant..." Hence D is the correct option.
Options A, B and C are incorrect as they do not relate to the sentence.