**Core Concept**
Hydrocolloids are a type of impression material used in dentistry for making accurate impressions of the teeth and surrounding tissues. Their flexibility is a critical property that affects their ability to capture the details of the oral cavity.

**Why the Correct Answer is Right**
The flexibility of hydrocolloids is due to their gel-like consistency, which is a result of the interaction between the hydrophilic (water-loving) and hydrophobic (water-repelling) components of the material. When a hydrocolloid is mixed with water, the hydrophilic molecules absorb the water and swell, creating a gel-like state. This gel-like consistency allows hydrocolloids to flow and conform to the details of the oral cavity, making them a popular choice for impression materials.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hydrocolloids are actually known for their elastic recovery, not rigidity. When subjected to stress, hydrocolloids can return to their original shape, making them a good choice for impression materials.
* **Option B:** Hydrocolloids do not exhibit high rigidity, as they are designed to be flexible and conformable.
* **Option C:** Hydrocolloids are not known for their low flexibility, but rather for their ability to flow and conform to the details of the oral cavity.

**Clinical Pearl / High-Yield Fact**
A key characteristic of hydrocolloids is their ability to capture the details of the oral cavity, making them a popular choice for impression materials. However, they can be sensitive to temperature and humidity changes, which can affect their accuracy.

**Correct Answer: B. Hydrocolloids are flexible impression materials.**