**Core Concept**
The question is about the composition of Buckley's formocresol, a medicament used in dental procedures, particularly in root canal treatments and pulpotomies. **Formocresol** is a mixture of formaldehyde and cresol, and its concentration is crucial for its effectiveness and safety.

**Why the Correct Answer is Right**
Buckley's formocresol contains 19% formaldehyde, which is the active ingredient responsible for its antimicrobial and tissue-fixing properties. This concentration is critical for the medicament's intended use, as it must be potent enough to achieve the desired therapeutic effect without causing undue harm to the patient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the concentration of formaldehyde in Buckley's formocresol is not this value.
**Option C:** Similarly, this option does not accurately reflect the formaldehyde concentration in Buckley's formocresol.
**Option D:** This option is also incorrect, as it does not match the known concentration of formaldehyde in the medicament.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Buckley's formocresol contains 19% formaldehyde, as this information is critical for safe and effective use in dental procedures.

**Correct Answer:** Correct Answer: B. 19%