## Core Concept
Glucose is an aldohexose with four chiral centers (asymmetric carbon atoms) in its open-chain form. The number of optical isomers for a compound is determined by the formula 2^n, where n is the number of chiral centers.

## Why the Correct Answer is Right
The structure of glucose has four chiral centers. Applying the formula 2^n, where n = 4, we get 2^4 = 16. This means glucose can exist as 16 different optical isomers. These isomers include D-glucose and its various enantiomers and diastereomers.

## Why Each Wrong Option is Incorrect
* **Option A:** This option suggests 2 optical isomers, which would be correct for a compound with only 1 chiral center (2^1 = 2). Glucose has more than one chiral center, making this option incorrect.
* **Option B:** This option suggests 4 optical isomers, which would be correct for a compound with 2 chiral centers (2^2 = 4). Since glucose has 4 chiral centers, this option is incorrect.
* **Option C:** This option suggests 8 optical isomers, which would be correct for a compound with 3 chiral centers (2^3 = 8). Given that glucose has 4 chiral centers, this option is also incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that glucose, being an aldohexose, exists primarily in its D-form in nature. The concept of optical isomers is crucial in biochemistry, especially when understanding the metabolism and biological activity of sugars.

## Correct Answer Line
**Correct Answer: D. 16.**