Final answer:
A grub is an insect, specifically the larval stage of beetles, which undergo complete metamorphosis. Slugs and nematodes are not insects; slugs are mollusks and nematodes are roundworms.
Step-by-step explanation:
The question is asking which of the given options is classified as an insect. Based on the characteristics of insects, the correct answer is c) grub. Insects are distinguishable by traits such as having a body divided into three main parts (head, thorax, abdomen), six legs, compound eyes, and two antennae. They belong to the phylum Arthropoda and the class Insecta, also known as Hexapoda.
A slug is not an insect; it is a mollusk. A nematode is also not an insect; it is a roundworm and belongs to the phylum Nematoda. A grub is the larval stage of certain insects, such as beetles, and therefore is an insect. Insects like grubs undergo various forms of metamorphosis during their development.
To elaborate, incomplete metamorphosis, as seen in grasshoppers, involves a gradual change from the immature nymph to the adult form, while complete metamorphosis, which occurs in butterflies, involves distinct larval, pupal, and adult stages.
Therefore, the correct identification would be that a grub is an insect because of its developmental stage in the insect lifecycle, specifically the larval stage of beetles, which undergo complete metamorphosis.