**Core Concept**
The glycemic index (GI) is a measure of how quickly the carbohydrates in a particular food raise blood sugar levels. It is an important concept in the management of diabetes and for individuals following a low-carb diet. The GI is calculated based on the area under the blood glucose curve after consuming a particular food compared to a reference food, usually white bread or glucose.

**Why the Correct Answer is Right**
The correct answer is **C. Fructose**. Fructose is a simple sugar that is naturally present in fruits, honey, and some vegetables. It is metabolized differently than glucose and does not stimulate a significant increase in insulin secretion. As a result, fructose has a low glycemic index. This is because fructose is primarily metabolized in the liver, where it is converted into glucose and then used for energy or stored as glycogen. The low GI of fructose makes it a good choice for individuals with diabetes or those who want to manage their blood sugar levels.

**Why Each Wrong Option is Incorrect**
**Option A:** **Glucose** - Glucose is a simple sugar that is rapidly absorbed into the bloodstream, causing a significant increase in blood sugar levels. As a result, glucose has a high GI, making it a poor choice for individuals with diabetes or those who want to manage their blood sugar levels.

**Option B:** **Starch** - Starch is a complex carbohydrate that is broken down into glucose during digestion. While starch has a lower GI than glucose, it is still a significant source of carbohydrates and can cause a rapid increase in blood sugar levels.

**Option C:** **Fructose** - (Already explained above)

**Option D:** **Lactose** - Lactose is a disaccharide sugar found in milk and other dairy products. While lactose has a lower GI than glucose, it is still a significant source of carbohydrates and can cause a moderate increase in blood sugar levels.

**Clinical Pearl / High-Yield Fact**
When managing diabetes or blood sugar levels, it is essential to choose foods with a low GI, such as non-starchy vegetables, fruits, and whole grains. Fructose, in particular, is a good choice due to its low GI and minimal impact on insulin secretion.

**Correct Answer: C. Fructose**