A) Among animals, insects are the most species-rich taxonomic group, making up more than 70 percent of the total. That means, out of every 10 animals on this planet, 7 are insects.
B) Other animals occupy the second position.
C) Molluscs are third among the group of invertebrates.
D) Crustaceans are least in number.
So, the correct answer is 'a- Insects, b- Molluscs, c- Crustaceana, d- Other animal groups'.