Fluctuative
Since the abolition of fuel subsidies, prices have been fluctuative.
Nice try! The IELTS examiner will understand that you are trying to make an adjective from the verb fluctuate using ive. Normally this would be a good strategy, but there is no such word as fluctuative!
The safest approach is to use the verb form:
Since the abolition of fuel subsidies, prices have fluctuated.
Alternatively, you can make an adjective phrase using the noun form and featuring some collocation:
Since the abolition of fuel subsidies, prices have been subject to fluctuation.
Finally you might try some even more sophisticated academic collocation:
Since the abolition of fuel subsidies, prices have tended to fluctuate.
Now go ahead and remove fluctuative from your list of ‘ive’ adjectives!