What is the unstressed vowel in word frightening?

The unstressed vowel in "frightening" is the "i" in the first syllable, "fright".

Here's why:

* Stress: The word "frightening" has the stress on the second syllable, "en".

* Unstressed vowels: Unstressed vowels in English often have a reduced sound, often close to the sound of the "schwa" (ə).

* "i" sound: In "frightening," the "i" in "fright" is pronounced with a shorter, less distinct sound than the stressed "i" in "en".

So, while it's still an "i", it's an unstressed vowel with a reduced sound.