In simple terms, scrubbing the stain only made it worse. Exacerbating means to make something worse than what it already is.
What book is this from? Can't help without knowing.
An ambiguous reference means that it's vague, and it leaves the reader unsure. this often occurs when a pronounce is set after two or more subjects that it COULD apply to, but the reader isn't sure. your answer is choice 1.
