A neonatally thymectomized mouse, immunized with protein antigen shows (A) both primary and secondary responses to the antigen (B) only primary response to the antigen (C) delayed type hypersensitive reactions (D) no response to the antigen

Answer: (B) only primary response to the antigen
A thymectomized mouse lacks mature T cells, so it cannot mount a secondary immune response which requires T cell help. It may still show a primary response to the antigen but lacks the ability to mount a secondary response.