hello
We use 'an' before the single noun or adjective when the first letter is a,o,e,i .
example : I had an offensive criticism yesterday .
You are right generally. Offensive does begin with a vowel sound. However, criticism is not a 'countable' noun in English. So, you can't receive 'a criticism'. I would say:
I got/received some offensive criticism yesterday.![]()
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