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’ and so this time you will receive a low score for vocabulary.
The safest approach is to use the verb form:
Alternatively, you can make an adjective phrase using the noun form and featuring some collocation:
Finally you might try some more fancy academic collocation:
Now go ahead and remove fluctuative from your list of ‘ive’ adjectives!