Final answer:
The tense vowels are represented by the sounds /ē/ and /yū/, which are more elongated and involve a tighter constriction of the vocal tract, making option 'a' the correct answer.
Step-by-step explanation:
Classification of Tense Vowels
Linguists classify vowel sounds based on their articulation characteristics, such as tension. Tense vowels are typically longer in duration and require more muscular effort and a more extreme position of the tongue within the mouth. The options given include different vowel sounds, but tense vowels are specifically represented by the sounds /ē/ and /yū/ as in the words 'see' (spelled with ) and 'duty' (spelled with ). These vowels are more elongated and involve a tighter constriction of the vocal tract than their lax counterparts.
Other vowel sounds, like /ǝ/ (schwa) and /ŏ/ for example, would be classified as lax vowels because they are typically shorter in duration and less muscularly tense. Understanding the difference between tense and lax vowels is essential in the study of phonetics and phonology, branches of linguistics that deal with the sounds of speech.