Laminarin and mannitol, the reserve food of brown algae, are a. lipids b. complex carbohydrates c. proteins d. lipoproteins

Options

A. lipids
B. complex carbohydrates
C. proteins
D. lipoproteins.

Explanation

To solve the question regarding the nature of laminarin and mannitol, the reserve food of brown algae, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the substances**: The question mentions two substances - laminarin and mannitol. We need to analyze what these substances are.

2. **Understand mannitol**: Mannitol is described as a sugar alcohol derived from mannose. This means it is a type of carbohydrate, specifically a sugar.

3. **Understand laminarin**: Laminarin is identified as a polysaccharide that is linear in structure. It is a complex carbohydrate made up of glucose units linked together, particularly with beta-1,3 linkages.

4. **Categorize the substances**: Since both mannitol (a sugar) and laminarin (a polysaccharide) fall under the category of carbohydrates, we need to determine if they are simple or complex carbohydrates. Mannitol, being a sugar alcohol, is a simple carbohydrate, while laminarin, being a polysaccharide, is a complex carbohydrate.

5. **Evaluate the options**: The options provided are:
- a. lipids
- b. complex carbohydrates
- c. proteins
- d. lipoproteins

Since we established that both laminarin and mannitol are carbohydrates, and laminarin is specifically a complex carbohydrate, we can conclude that the correct answer is option B.

6. **Final conclusion**: Therefore, the reserve food of brown algae, laminarin and mannitol, are classified as complex carbohydrates.

### Final Answer:
**b. complex carbohydrates**