Final answer:
Tense vowels, like /ē/ and /yū/, involve greater muscle contraction and longer duration; hence, option a is correct.
Step-by-step explanation:
In the study of phonetics and phonology, linguists classify vowels based on several features, one of which is tenseness. Tense vowels are generally characterized by a greater muscle contraction and a longer duration than lax vowels. They often occur in open syllables (syllables that end with the vowel sound) and can be sustained longer while keeping the same vowel quality. Considering the options listed, the correct answer would be a. /ē/ and /yū/, as these vowels sound tense. For example, the /ē/ sound can be found in words like 'see' (with the spelling ea), and the /yū/ sound can be found in the word 'you'. In contrast, options b, c, and d feature lax vowels.