The correct answer is a, b and d. Explanation & Key Takeaways: • a. correct. Phylum Mollusca is the second largest phylum in the animal kingdom, second only to Arthropoda in terms of the number of described species. • b. Correct. Molluscs are generally bilaterally symmetrical. However, this symmetry can be lost in some groups over the course of development, such as in gastropods (snails and slugs). • c. Incorrect. Molluscs are not only terrestrial; they can be found in a variety of environments including freshwater and marine environments. For example, clams, oysters, octopuses, and squids are all molluscs that live in aquatic environments. • d. Correct. One of the characteristic features of molluscs is the presence of a muscular foot, which can be used for movement, attachment, or a combination of functions depending on the species. • Therefore, statements a, b, and d are correct.