**Core Concept**
Cardiac index is a measure used to assess the cardiac function in relation to the body size. It is a dimensionless quantity that takes into account the cardiac output and the body surface area. This concept is crucial in evaluating the heart's pumping efficiency in different patients, especially those with varying body sizes.

**Why the Correct Answer is Right**
The cardiac index is calculated by dividing the cardiac output (the volume of blood the heart pumps per minute) by the body surface area. The body surface area is an estimate of the total area of the body that the cardiac output is supplying with blood. This ratio helps to normalize the cardiac output for different body sizes, allowing for a more accurate comparison of cardiac function between individuals. The cardiac index is expressed in units of mL/min/m² and is a critical parameter in assessing cardiac performance, especially in patients with heart failure or other cardiovascular diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** Cardiac output and body weight is not a correct ratio for calculating the cardiac index. Body weight is not a direct measure of the body's surface area, which is what the cardiac index takes into account.
**Option C:** Cardiac output and work of heart is not a relevant ratio for calculating the cardiac index. The work of the heart is not a standard physiological parameter used in calculating cardiac output or cardiac index.
**Option D:** Surface volume and surface area is not a correct ratio for calculating the cardiac index. Surface volume is not a standard physiological parameter used in calculating cardiac output or cardiac index.

**Clinical Pearl / High-Yield Fact**
Remember that the cardiac index is an important parameter in assessing cardiac function, especially in patients with heart failure or other cardiovascular diseases. A low cardiac index indicates poor cardiac performance, while a high cardiac index may indicate a hyperdynamic circulation.

**✓ Correct Answer: B. Cardiac output and surface area**