Quellung phenomenon is seen in:

Ans. is 'a' i.e., Pneumococcus (Ref: Ananthanarayan, 8th/e, p. 220, 9th/e, p. 223)* A positive reaction shows clumping because of cross-linking of the antibodies and pneumococci. The halo effect around the pneumococci is apparent capsular swelling. A negative control would show no clumping or capsular swelling.