The second largest phylum is (a) Arthropoda (b) Chordata (c) Echinodermata (d) Mollusca

We need to know the species diversity and a little background about each of the given phyla in order to answer the question. Let's take a brief look: (a) Arthropoda - This phylum contains the largest number of described species (more than 1 million), and it includes animals like insects, spiders, and crustaceans. (b) Chordata - This phylum includes vertebrates (animals with a backbone) like mammals, birds, reptiles, amphibians, and fishes, as well as some invertebrates like sea squirts and lancelets. This phylum has around 65,000 species. (c) Echinodermata - This phylum includes sea stars, sea cucumbers, and sea urchins, and it has around 7,000 species. (d) Mollusca - This phylum includes animals like snails, clams, squids, and octopuses, with around 85,000 known species.

Now that we have information about the species diversity in each phylum, we can rank the phyla in descending order of species diversity: 1. Arthropoda (over 1 million species) 2. Mollusca (around 85,000 species) 3. Chordata (around 65,000 species) 4. Echinodermata (around 7,000 species)

From the ranking in Step 2, we see that Mollusca (option d) is the second largest phylum in terms of species diversity.

The correct answer is (d) Mollusca, the second largest phylum in terms of species diversity.