How long after a booster rabies vaccination is a cat (or dog, or ferret) considered to be currently vaccinated and protected against rabies?

Sorry.  You chose C, but the correct answer is A. Immediately

According to the 2016 Compendium for Rabies Control, a peak rabies virus antibody titer is reached 28 days after initial vaccination and immediately after booster vaccination.

Here is some additional information on rabies:
When in doubt, it is never wrong to check with your local health department.

Basically all potential rabies exposures boil down to 2 questions:
1. Who is involved?
Animal-animal exposure (less alarm bells)
Animal bites/exposed human (more alarm bells)

2. Was dog/cat/ferret vaccinated?
--Documented up-to-date on rabies vaccination (booster + 45 days owner observation)
--Documented vaccinated, but overdue (booster + 45 days owner observation)

--UN-documented vaccinated, and overdue (Handle on case-by-case basis)
--Ferrets that are OVERDUE for booster (Handle on case-by-case basis)

--UN-vaccinated pet (euthanize or vaccinate + 4-6 month strict isolation)
--Wild animal, esp. bats, raccoon, skunk (euthanize, send head to state lab)

When dealing with rabies questions, ask yourself if this seems like a HIGH-risk exposure (e.g., wild raccoon bites a child)
or a LOWER risk exposure (e.g., up-to-date vaccinated dog messes with woodchuck but no bite wounds on dog).

For high-risk lean towards euthanasia/testing or immediate vaccination + long, strict isolation (4 months, dogs/cats, 6 months, ferrets).
For low -isk lean towards immediate vaccination + shorter, easier observation period (45 days).

In general, both Canadian and U.S. guidance on rabies post exposure management conforms with Compendium guidelines.

United States guidelines
2016 Compendium for Rabies Prevention and Control, JAVMA, Vol 248, No. 5., 505-517, courtesy, Natl. Assoc. State Public Health Veterinarians (NASPHV compendia).
For information on post-exposure prophylaxis in people see: Human Rabies Prevention-US, 2008 ACIP Reccs: May 23, 2008 / 57(RR03);1-26,28.

Canadian guidelines
Canadian Food Inspection Agency (CFIA) rabies home page and rabies testing summary.
The Canadian Veterinary Medical Association (CVMA) on rabies guidance homepage.
Click here for a CVMA post-exposure management summary.
Click here for CVMA post-exposure management presentation.

Global guidelines
WHO OIE World Health Organization Expert Consultation on Rabies (2nd Report, 2013).