**Core Concept**
The vaginal sponge is a type of barrier contraceptive device designed for women, which requires insertion before intercourse to prevent sperm from reaching the egg. Its effectiveness depends on proper use and maintenance.

**Why the Correct Answer is Right**
The correct answer, 9%, reflects the estimated failure rate of the vaginal sponge when used correctly. This is attributed to the sponge's design, which may not fit snugly in some women, or if it is not left in place for the recommended 6 hours after intercourse. The sponge's effectiveness also decreases if it is not used correctly, such as if it is inserted too late or if it is not removed within 24 hours. The failure rate is further influenced by factors such as the user's age, body weight, and the presence of any medical conditions that may affect the user's ability to use the sponge correctly.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% is an overly optimistic estimate, as it does not account for the various factors that can lead to the sponge's failure, such as improper insertion or removal.

**Option C:** 16% is a higher failure rate than what has been reported in clinical studies, suggesting that it is an overestimation.

**Option D:** 20% is an even higher failure rate than Option C, which is unlikely given the available data on the vaginal sponge's effectiveness.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the vaginal sponge's failure rate can be significantly higher if it is not used correctly, highlighting the importance of proper education and training for users.

**✓ Correct Answer: B. 9%**