**Core Concept**
The Henderson-Hasselbalch equation is a fundamental concept in acid-base chemistry, describing the relationship between the pH of a solution, the pKa of a weak acid, and the concentrations of the acid and its conjugate base (salt). In this context, the pKa is a critical value that determines the ionization state of the weak acid.

**Why the Correct Answer is Right**
At the pH equal to the pKa, the equilibrium between the weak acid (HA) and its conjugate base (A-) is at its midpoint. According to the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), when pH = pKa, the log term equals zero, and the ratio of the concentrations of the salt (A-) and the acid (HA) is equal to 1. This is because the logarithm of 1 is 0, and the equation simplifies to pH = pKa.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it represents a ratio that is not equal to 1 when pH = pKa. A ratio of 1:10 or 10:1 would be more representative of a solution with a pH significantly different from the pKa.
**Option B:** This option is incorrect because it implies a ratio that is not equal to 1 when pH = pKa. A ratio of 10:1 or 1:10 would be more representative of a solution with a pH significantly different from the pKa.
**Option C:** This option is incorrect because it implies a ratio that is not equal to 1 when pH = pKa. A ratio of 1:100 or 100:1 would be more representative of a solution with a pH significantly different from the pKa.

**Clinical Pearl / High-Yield Fact**
A key takeaway from the Henderson-Hasselbalch equation is that at pH = pKa, the concentrations of the acid and its conjugate base are equal. This concept is crucial in understanding acid-base chemistry and its applications in various medical contexts.

**Correct Answer:** .