Choose the one word that can correctly replace the description: Animals without a backbone.

Introduction / Context:
This one word substitution question tests basic biology terminology. In zoology, animals are broadly divided into vertebrates and invertebrates based on the presence or absence of a backbone or vertebral column. Knowing these terms helps in school science as well as in general knowledge exams. The question asks for the scientific term for animals that lack a backbone.

Given Data / Assumptions:

• The animals described do not have a backbone or vertebral column.
• The options are marsupials, mammals, vertebrate, and invertebrates.
• The term must be applicable to a wide range of animals such as insects, worms, and many sea creatures.
• We assume standard school level biology definitions.

Concept / Approach:
Vertebrates are animals with a backbone. The prefix in invertebrates indicates absence. Therefore, invertebrates are animals without a vertebral column. Marsupials are a special group of mammals that carry their young in a pouch. Mammals form a class of vertebrate animals with features like hair and milk production. The option vertebrate is clearly the opposite of what is described. Hence, invertebrates is the correct scientific term.

Step-by-Step Solution:
Step 1: Recall that the key feature mentioned is absence of a backbone. Step 2: Remember that vertebrate animals have a backbone and form a specific group including fish, amphibians, reptiles, birds, and mammals. Step 3: Apply the negative prefix in to vertebrate to form invertebrate, meaning animals without a backbone. Step 4: Look at option d, invertebrates, and see that it exactly matches this definition. Step 5: Reject marsupials because they are a subgroup of mammals that carry offspring in a pouch and still possess backbones. Step 6: Reject mammals because they are vertebrate animals and therefore do have backbones. Step 7: Reject vertebrate because it directly contradicts the description of animals without a backbone.

Verification / Alternative check:
Think of examples. Insects, spiders, earthworms, starfish, and jellyfish do not have backbones. These are called invertebrates in school textbooks. On the other hand, dogs, cows, and humans have backbones and are vertebrates. This comparison confirms that invertebrates is the correct term for animals without a backbone.

Why Other Options Are Wrong:
Marsupials is wrong because it refers to a specific group of mammals like kangaroos and koalas, which possess backbones. Mammals is wrong since mammals are vertebrates and the question specifically mentions absence of a backbone. Vertebrate is wrong because it states the opposite condition, namely presence of a backbone.

Common Pitfalls:
Students sometimes confuse vertebrates and invertebrates due to the similar spelling. Another mistake is not paying attention to the negative prefix in and mixing up the two terms. A simple memory aid is to remember that vertebrate and vertebra share the same root, while invertebrate simply adds a negative prefix indicating absence of a vertebral column.

Final Answer:
The correct one word term for animals without a backbone is invertebrates.