15. Luigi's motor moves slowly as we venture along the trail. I can hear the gravel quietly crunching beneath Luigi's rubber wheels.

the correct answer is:

from context clues heliotrope is probably a flower.

a heliotrope are peruvian plants that have small purplish plants.

the writer needs to avoid using abbreviations and check spelling before sharing.

see explanation.

explanation:

1. buoyant: happy and confident (e)

2. euphoric: in a state of great happiness (a)

3. jubilant: having a feeling of great happiness and excitement (b)

4. rapturous: feeling or expressing great joy (d)

*note that the difference in 1-4 can be shown by signal words such as "in a state of" for the term "euphoric," as "euphoric" would describe a state of euphoria.

5. untroubled: calm and tranquil (c)

6. cajole: to talk coaxingly and flatteringly (i)

7. harangue: to express oneself at considerable length (h)

8. quibble: to evade the truth of the point at issue by arguing over trifles (g)

9. whine: to complain in a childish, fretful manner and in a nasal tone (j)

10. rant: to talk bombastically (f)

i agree, i think a best suits this answer.