Select the synonym of the word spine, meaning the backbone or the series of vertebrae extending from the skull to the lower back.

Introduction / Context:
This vocabulary question asks for a synonym of the word spine in a biological or anatomical sense. The spine is commonly understood as the backbone, the central support structure in the human body and many animals. The options contain words from different semantic fields, and only one is closely related to this anatomical meaning.

Given Data / Assumptions:

• Target word: spine.
• Options: supple, vertebrae, rotund, grime.
• We focus on the physical, anatomical meaning of spine, not metaphorical uses like courage or determination.
• We assume basic knowledge of body parts and common scientific terms.

Concept / Approach:
Anatomically, the spine is composed of a series of small bones called vertebrae that run from the neck down to the lower back. Thus, vertebral column and spine are closely related, and vertebrae are the individual bones that together form the spine. Among the options, vertebrae is therefore the closest to spine in meaning. The other options describe flexibility, shape, or dirt, and are clearly unrelated to the anatomical structure being tested.

Step-by-Step Solution:
Step 1: Recall that the spine is also called the backbone or vertebral column and consists of many small bones. Step 2: Examine vertebrae. This is the plural of vertebra, the individual bone that forms part of the spinal column. Step 3: Notice that spine and vertebrae are directly connected in meaning, with vertebrae being the components of the spine. Step 4: Examine supple. This word means flexible or easily bent, usually describing muscles, leather, or movement, not a specific body part. Step 5: Examine rotund. This adjective means round or plump in shape, again not connected to the spine. Step 6: Examine grime. This is a noun meaning dirt or soot, completely unrelated to anatomy. Step 7: Conclude that vertebrae is the only option that can serve as a near synonym for spine in this context.

Verification / Alternative check:
If you open any basic biology textbook, you will find explanations like The human spine is made up of 33 vertebrae. This shows that spine and vertebrae are used together to talk about the same body region. While they are not exact one word replacements in all sentences, vertebrae is clearly the closest match in meaning among the options given. None of the other words ever appear in definitions of spine.

Why Other Options Are Wrong:
Supple is wrong because it describes a property (flexibility) and does not name a body structure. Rotund is wrong because it describes roundness or plumpness in shape and is unrelated to the backbone. Grime is wrong because it refers to dirt, often on surfaces or skin, and has no connection to the spine as an anatomical term.

Common Pitfalls:
Some students may be distracted by supple because it is vaguely associated with movement and the body. However, the question asks for a synonym of spine, not a word describing how the spine might move. Another pitfall is to overthink and reject vertebrae because it is the plural form, but vocabulary questions often accept closely related terms like this as synonyms when they point to the same concept area.

Final Answer:
The synonym of spine in this context is vertebrae.