Final answer:
The knockout of the gene Bcl-6 results in the inability to form germinal centers, which are crucial for B cell development and the immune response.
Step-by-step explanation:
The gene whose knockout results in the inability to form germinal centers is D. Bcl-6. Bcl-6 is a transcriptional repressor that is critically important for the formation of germinal centers during B cell development. Germinal centers are the sites within secondary lymphoid organs where mature B cells proliferate, differentiate, and undergo somatic hypermutation and class-switch recombination, which are essential processes for the development of high-affinity antibodies and immunological memory. A knockout of Bcl-6 disrupts these processes, leading to the impairment of humoral immunity and the inability to form germinal centers.