**Core Concept**
The yellow fever vaccine is a live, attenuated vaccine that provides immunity against **yellow fever virus**. It is administered via intramuscular injection, and its efficacy depends on the body's immune response. The vaccine's protective effects are primarily due to the induction of **neutralizing antibodies**.

**Why the Correct Answer is Right**
The yellow fever vaccine starts providing protection approximately 10 days after injection, as this is the time required for the vaccine to induce a sufficient immune response. This includes the production of **neutralizing antibodies** and the activation of **cell-mediated immunity**. The exact timing can vary, but 10 days is the generally accepted timeframe for the onset of protection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests an onset of protection that is too early, not allowing sufficient time for the immune system to respond to the vaccine.
**Option B:** This option is incorrect because it suggests an onset of protection that is too late, potentially leaving the individual vulnerable to infection for an extended period.
**Option D:** This option is incorrect because it suggests an onset of protection that is too late, which is not supported by the immunological response to the vaccine.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the yellow fever vaccine is highly effective, with a **seroconversion rate** of over 90% in individuals who receive the vaccine. However, the vaccine's protective effects may wane over time, and **booster doses** may be necessary for individuals who remain at risk of infection.

**Correct Answer:** C. 10 days