The correct option is B Sensitive
'Acutely affected by external impressions' is called Option D, Sensitive. Option A, 'Ingenious' means innovative or original. Option B 'Innocent' is the one who is pure or not guilty. Option C, 'Credulous' is the one who willingly believes what others say and can be easily deceived. So, options A, B and C are incorrect and the correct word for the phrase is Option D, Sensitive.