"Exacerbate" means to make a problem worse, and "mitigate" means to make it less severe, so the pair is a pair of opposites: worsen versus ease.
Only "Aggravate : Alleviate" repeats the same worsen-and-ease opposition as "Exacerbate : Mitigate", so option (A) is correct.
The equation \[ y'' + p(x)y' + q(x)y = r(x) \] is a _________ ordinary differential equation.