Alleviate means to make something less severe, to ease, relieve, or reduce suffering/pain. The opposite would be to make something worse or more severe. 'Aggravate' means to annoy or make a problem/situation worse, which is the correct antonym. 'Incubate' means to develop slowly, 'Improvise' means to create without preparation, and 'Arrogate' means to claim without right - none are opposites.