The correct option is (a) having no backbone.
Invertebrates are the category of organisms that lack backbone. They have some soft tissues and because of this their body is soft as compared to that of vertebrates. Example: jellyfish and sponges. Some of the invertebrates have outer covering which is very hard called as exoskeleton. This helps them to protect from outer injuries and harsh conditions. Example: cockroach, spiders, etc.