An invertebrate is an animal that does not have a backbone or spinal column. Invertebrates make up the majority of animal species on Earth and can be found in a wide range of habitats, including the ocean, forests, and even in our own backyards.

The correct answer to the question is option 'D', which is the butterfly. Let's explain why the butterfly is an example of an invertebrate.

- Characteristics of Invertebrates:
Invertebrates can vary greatly in size, shape, and behavior, but they all share the common characteristic of lacking a backbone. Some key characteristics of invertebrates include:
1. No Backbone: Unlike vertebrates (animals with backbones), invertebrates do not possess a spinal column made up of individual vertebrae.
2. Exoskeleton or Soft Body: Invertebrates may have a hard exoskeleton made of chitin, like insects, or a soft body, like squids or jellyfish.
3. Diverse Forms: Invertebrates exhibit a wide range of body forms, including worms, insects, spiders, mollusks, and more.

- The Example of a Butterfly:
Butterflies are insects, and insects are a type of invertebrate. Here's why butterflies fit the criteria of an invertebrate:
1. Lack of Backbone: Butterflies do not have a backbone. Instead, they have a segmented body supported by an exoskeleton.
2. Exoskeleton: Like all insects, butterflies have an exoskeleton, which is a hard outer covering that protects and supports their bodies. The exoskeleton is made of chitin and provides structure and protection for the butterfly.
3. Jointed Legs and Segmented Body: Butterflies have six jointed legs and a segmented body, both of which are common characteristics of insects.

In conclusion, the butterfly is an example of an invertebrate because it lacks a backbone, has an exoskeleton, and possesses other key characteristics of insects. It is important to note that other options given in the question, such as the snake, frog, and fish, are all examples of vertebrates as they have backbones.