Correct option (D) Harmful
Explanation:
Deadly : It means 'Fatal'.
Hence, this is not a proper antonym to 'innocuous'.
Ferocious : It means 'horrible'
Hence, irrelevant to the opposite of 'innocuous'.
Poisonous : It means 'venomous'.
Hence, an irrelevant 'antonym'.
Harmful : It is a perfect antonym of innocuous which itself means 'harmless'.