An algae used as chickening and gelling agents in food is _______ .

Explanation

**Step-by-Step Solution:**

1. **Identify the Type of Organism**: The question asks for an algae that is used as a thickening and gelling agent in food. This indicates that we are looking for a specific type of algae.

2. **Recall Common Algae Used in Food**: In the food industry, certain algae are known for their thickening and gelling properties. One of the most common types is red seaweed.

3. **Name the Specific Algae**: The specific algae that is widely recognized for its use as a thickener and gelling agent is called "carrageenan." It is derived from red seaweed, particularly from a species known as Irish moss.

...