The most critical virulence factor shared by Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae, which cause the vast majority of bacterial meningitis cases, is the presence of a capsule.
A virulence factor is a component or characteristic of an organism that contributes to its ability to establish itself in a host and cause disease. In the case of these three organisms, the capsule is an outer layer that protects the bacteria from the host's immune system, thereby allowing them to establish and spread the infection.
Protein A, endotoxin, and β-lactamase, while also virulence factors, are not the MOST critical in these specific organisms' ability to cause bacterial meningitis.
Therefore, the correct answer to the question "What is the MOST important pathogenic component they share?" is (B) Capsule.