Hi jooney, if the speaker knows what is being cooked he/she she is more likely to say "The steak she is cooking smells delicious" or "The lasagne she is cooking smells delicious" if he/she doesn't know then yes B is the correct way of saying it.
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jooneyGoing back to the original example, if the speaker does not know what she is cooking in "A" and utters that sentence, why does he choose 'A' over "B"? I mean, if they both convey the same message, what prompts him to choose "A" over "B"?I have no idea!