The question is not clear. You have to explain exactly what you want to do with this. However, looking at those options, I’m guessing that the required answer is C.
If ax² + b = 0, and assuming this means that b is positive,
then ax² = -b
x² = -b/a
Since x² must be positive, this means that a must be negative if x² has real roots.
So ab < 0
If the roots of ax^2 + b = 0 are what?
ax^2 + b = 0
x^2 = -b/a
x = ±√(-b/a)
Answer is a > 0, b ≤ 0 OR a < 0, b ≥ 0