## Core Concept
The question pertains to the physical properties of substances, specifically their densities. Density is defined as mass per unit volume of a substance. The substances in question are not explicitly named, but based on common knowledge of materials science and physics, we can infer that the question relates to the comparison of densities of various materials or elements.

## Why the Correct Answer is Right
Without specific details on the substances represented by , , , and , we must rely on general principles. Typically, when comparing densities, we consider well-known values:
- Lithium (Li) has a density of approximately 0.534 g/cm³.
- Water has a density of 1 g/cm³.
- Most metals have densities greater than that of water, with some exceptions like lithium and sodium.

## Why Each Wrong Option is Incorrect
**Option A:** Assuming represents a common substance with a density higher than that of water or the correct answer, its higher density makes it incorrect for the context of the question asking for the "least density."

**Option B:** If represents water or a substance with a density close to or higher than water, it would not be the correct answer if there's a substance listed with a lower density.

**Option D:** Similarly, if represents a substance with a density higher than that of , it would not be the correct answer.

## Clinical Pearl / High-Yield Fact
A high-yield fact to remember is that lithium, with a density of about 0.534 g/cm³, is one of the lightest metals and has one of the lowest densities among elements, which can be a useful reference point in physics and chemistry.

## Correct Answer: .