Answer: A. 10%
On average, influenza cases caused by B strains account for roughly 25% of the total cases of flu. If we assume the trivalent will have the wrong B strain in it half the time, then, assuming perfect efficacy, the quadrivalent with 4 strains should be about 12.5% better on average.
So, why doesn't the ACIP/CDC issue a preference for the quadrivalent? For reasons I will guess at below, the ACIP is hesitant to preferentially recommend one vaccine over another. Another example of this is the use of high-dose Fluzone over regular dose Fluzone in the 65-years-of-age-and-older group. Studies have shown not only significantly higher antibody titers with the high-dose vaccine, but also higher efficacy in preventing the flu as compared to the standard dose. Again, the ACIP does not voice a preference for the better vaccine.
Someone more cynical than I might argue that the ACIP doesn't want to recommend one vaccine over another for fear of driving the less effective vaccine out of the marketplace. Not only would this anger a major pharmaceutical company with its political clout, but would also likely result in higher vaccine prices for the remaining vaccine that now is without a competitor. Who is the major purchaser of vaccines? The Federal government. Another problem with having only one company making a vaccine is if there is a glitch in the manufacturing process that leads to decreased production, no one else can pick up production and the result is vaccine shortages. I suspect the ACIP also worries that some individuals may miss getting vaccinated if the “preferred” vaccine is not available while the “unpreferred” vaccine goes unused while providers and/or patients wait for the “better” vaccine.
So, the ACIP might feel under some constraints when it comes to recommending vaccines but you don't have to. Do what's best for your patient.
ACIP's official recommendations:
Grohskopf LA, Sokolow LZ, Olsen SJ, et al. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices, United States, 2015–16 Influenza Season. MMWR Morb Mortal Wkly Rep. 2015;64;818-825
This article looks at the High dose versus standard dose flu shot in the over 65 years of age cohort:
DiazGranados CA, Dunnning AJ, Kimmel J. Efficacy of high-dose versus standard-dose influenza vaccine in older adults. N Engl J Med 2014; 371:635–45.