The correct answer is option 'B' - Gaudikov's effect. This effect refers to the phenomenon of change in color of algae according to the depth in the sea. Let's understand this effect in detail:

**Gaudikov's Effect:**

Gaudikov's effect, also known as the "Vertical Zonation of Algae," describes the variation in the color of algae based on the depth they inhabit in the sea. This effect is primarily observed in marine environments where light penetration decreases with increasing depth.

**Reasons for the Change in Color:**

The change in color of algae is primarily attributed to the varying levels of light availability at different depths in the sea. Light is an essential requirement for photosynthesis, the process through which algae produce their food. Different wavelengths of light are absorbed by different pigments present in the algae, allowing them to carry out photosynthesis.

**Chlorophyll and Depth:**

Chlorophyll is the primary pigment responsible for photosynthesis in algae. It absorbs light in the red and blue regions of the electromagnetic spectrum and reflects green light, which gives many algae their characteristic green color. However, as light penetrates deeper into the sea, the red and blue wavelengths are absorbed more rapidly, while green light is scattered and reflected back. This results in a decrease in the presence of green algae and a shift towards other pigments.

**Carotenoids and Depth:**

Carotenoids are accessory pigments present in algae. They absorb light in different regions of the spectrum and play a role in capturing light energy, which is then transferred to chlorophyll for photosynthesis. As the depth increases, the concentration of carotenoids increases in algae. These pigments include xanthophylls, fucoxanthin, and phycobilins, which give algae a range of colors such as yellow, brown, and red.

**Other Factors:**

Apart from light availability, other environmental factors such as temperature, nutrient availability, and competition for resources can also influence the color and distribution of algae in the sea. However, Gaudikov's effect primarily focuses on the impact of light penetration and the resulting change in color of algae with depth.

**Conclusion:**

In conclusion, Gaudikov's effect explains the change in color of algae according to the depth in the sea. As light penetration decreases with increasing depth, the pigments present in algae, particularly chlorophyll and carotenoids, undergo changes to adapt to the available light conditions. This results in a shift in color from green to brown, yellow, and red. Understanding this effect aids in studying the distribution and diversity of algae in marine ecosystems.