Option 1: Ulothrix is a filamentous green algae that grows in freshwater environments. It is not an example of a parasitic alga.

Option 2: Cephaleuros is a parasitic alga that infects several plants, including tea plants, coffee plants, and mango trees. It causes orange or brownish patches on the leaves and stems of the host plants, affecting their growth and productivity. This makes option 2 the correct answer.

Option 3: Oedogonium is a filamentous green alga that is not parasitic. It is commonly found in freshwater ecosystems.

Option 4: Sargassum is a brown seaweed that is not parasitic. It is known for forming floating mats in the Sargasso Sea.

In conclusion, option 2 (Cephaleuros) is the correct example of a parasitic alga as it infects and harms host plants. Option 1 (Ulothrix), option 3 (Oedogonium), and option 4 (Sargassum) are not parasitic algae.