Most important source of infection for meningococci:: (A) Case, (B) Subclinical infection, (C) Carriers, (D) Latent case
Answer is C. During periods of endemic disease, about 10% of the general population harbour Neisseria meningitidis in the nasopharynx. 
Since N. meningitidis is a strict human pathogen and most patients have not been in contact with other cases, asymptomatic carriers are presumably the major source of the pathogenic strains.