Final answer:
The plural of 'vertebra' is 'vertebrae'. The given options do not provide the correct plural ending. Such change from '-a' to '-ae' is common in English for words of Latin or Greek origin, particularly in scientific terminology.
Step-by-step explanation:
The plural of the word vertebra in English is formed by transforming the singular form -a into the plural ending -ae. Therefore, the correct answer to your question is not provided among the given options a) -as, b) -en, c) -a, d) -us. The anatomical and medical field uses the terms vertebrae when referring to more than one vertebra, following a terminology rule adopted from Latin.
Such change in the noun ending is common in English for words of Latin or Greek origin, especially in the scientific terminology. A similar change is observed for other words ending in -a: 'alga' changes to 'algae'; 'larva' to 'larvae'; 'formula' to 'formulae' in British English (while American English often uses 'formulas'). However, it's important to note that not all English words which end with -a follow this rule. Regular English words usually take the -s or -es ending to signal the plural form.
It's always advisable to consult a good dictionary or reference book when in doubt, since English, like any other language, presents exceptions to its rules and presents varied linguistic influences.
Learn more about English plurals