## Core Concept
The respiratory quotient (RQ) is a measure used in physiology to describe the ratio of carbon dioxide (CO2) produced to oxygen (O2) consumed during the process of cellular respiration. It is an indicator of the type of food being metabolized by the body. The RQ varies depending on the substrate being metabolized: carbohydrates, fats, and proteins have different RQ values.

## Why the Correct Answer is Right
On a mixed diet, which typically consists of a combination of carbohydrates, fats, and proteins, the body's metabolic processes utilize these substrates in varying proportions. The RQ for carbohydrates is 1.0 (since glucose metabolism produces equal amounts of CO2 and consumes equal amounts of O2: C6H12O6 + 6O2 → 6CO2 + 6H2O), for fats it is approximately 0.7, and for proteins, it is about 0.8. A mixed diet, reflecting the average metabolic utilization of these substrates, results in an RQ that is generally close to 0.8. This value represents the average utilization of a mix of nutrients.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because an RQ of 0.7 would specifically indicate a diet high in fat, as fat metabolism yields an RQ of approximately 0.7.
- **Option B:** This option suggests an RQ of 0.9, which might be seen with a diet very high in protein or certain specific types of carbohydrate metabolism but is not typical for a standard mixed diet.
- **Option D:** This option suggests an RQ greater than 1, which could be seen with certain types of carbohydrate metabolism (like alcohol) but is not representative of a typical mixed diet.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that an RQ of 0.8 is often used as a general estimate for a mixed diet. This value can be important in critical care and anesthesiology for calculating caloric needs and understanding metabolic states. Remembering that RQ values can give insight into the types of substrates being metabolized can help in managing patients' nutritional needs.

## Correct Answer Line
**Correct Answer: C. 0.8**