The most common etiological agent for acute bronchiolitis in infancy is –: (A) Influenza virus, (B) Para influenza virus, (C) Rhinovirus, (D) Respiratory syncytial virus
Answer is D. "The most common etiological agent for Bronchiolitis is Respiratory syncytial virus. RSV is responsible for more than 50% of cases of bronchiolitis."

Bronchiolitis 

Bronchiolitis is the most common serious acute lower respiratory tract infection in infants and young children.
The most vulnerable group is between the ages of 1 and 6 months.
But the disease can affect children up to 2 years.
More common in males.

Organisms causing bronchiolitis

Bronchiolitis is predominantly a viral disease.

i) RSV (mostcommon) 
ii) Parainfluenza virus 3, 1, 2       
iii) Adenovirus    
iv) Influenza virus
v) Mycoplasma pneumoniae